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How big is the Estimated Size of the Universe?

The Estimated Size of the Universe is 1.6 null

Introduction.

Have you ever looked up at the stars on a clear night and wondered how big the universe is? You’re not alone. Scientists have been trying to measure the size of the universe for centuries. And while the exact size is still a mystery, we have a pretty good idea about the size of the observable universe - the part of the universe we can see from Earth. So, let’s embark on an exciting journey to understand the staggering size of the observable universe and how it compares to other objects we’re familiar with.

The Size of the Observable Universe

You might have heard that the universe is infinite, but when we talk about its size, we usually refer to the observable universe. That’s because, even though the universe might be infinite, there’s only a certain amount we can see from Earth. So, how big is the observable universe? Well, it’s about 93 billion light-years in diameter. To give you an idea of what that means, a light-year is the distance light travels in one year, and light travels at a speed of 186,282 miles per second. That’s like going around the Earth 7.5 times in just one second!

Size Comparisons

To better understand the size of the universe, let’s make a few comparisons:

• If you think of the Earth as a tiny grain of sand, then our entire solar system would be about the size of a quarter. In this comparison, the observable universe would be about the size of the Earth.
• If the observable universe were scaled down to the size of a football field, our Milky Way Galaxy would be the size of a grain of rice, and our solar system would be too small to see with the naked eye.
• If we compared the observable universe to the tallest building in the world, the Burj Khalifa, which stands at 828 meters high, our Milky Way Galaxy would only be 1 millimeter tall, and our solar system would be a thousand times smaller than a grain of sand.

Interesting Facts about the Universe’s Size

• The number of galaxies in the universe is astounding. Originally, scientists estimated that there might be 2 trillion galaxies in the observable universe. However, more recent data suggests the number is closer to several hundred billion. Still, that’s a lot of galaxies!
• The observable universe is centered on the observer. That means that every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.
• The term “observable” doesn’t mean what we can see with modern technology. It refers to the limit created by the speed of light itself. No signal can travel faster than light, so there’s a maximum distance beyond which we can’t detect anything, as the signals wouldn’t have reached us yet.

The size of the universe is beyond human comprehension. It’s so vast that we can only observe a tiny fraction of it from Earth. And even that observable universe is filled with hundreds of billions of galaxies, each containing billions of stars. It’s a testament to the power of nature and the wonder of scientific discovery. The next time you look up at the night sky, remember that you’re looking into an immense universe that extends far beyond what the eye can see.

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How big is the universe?

How big is the universe around us? What we can observe gives us an answer, but it's likely much bigger than that.

The observable universe

Measuring the universe, the shape of the universe, additional resources and reading, bibliography.

How big is the universe ? It's one of the fundamental questions of astronomy . By looking for the farthest observable point from Earth (and by extension, the oldest given the speed of light ) we can estimate a diameter. 

Thanks to evolving technology, astronomers are able to look back in time to the moments just after the Big Bang . This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. 

As a result, while we can make estimates as to the size of the universe scientists can't put a number on it. 

Related: What is the coldest place in the universe?

In 2013, the European Space Agency's Planck space mission released the most accurate and detailed map ever made of the universe's oldest light. The map revealed that the universe is 13.8 billion years old. Planck calculated the age by studying the cosmic microwave background .

"The cosmic microwave background light is a traveler from far away and long ago," said Charles Lawrence, the U.S. project scientist for the mission at NASA's Jet Propulsion Laboratory in Pasadena, California, in a statement . "When it arrives, it tells us about the whole history of our universe."

Because of the connection between distance and the speed of light, this means scientists can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word "observable" is key; the sphere limits what scientists can see but not what is there.

But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. If inflation occurred at a constant rate through the life of the universe, that same spot is 46 billion light-years away today according to Ethan Siegel, writing for Forbes , making the diameter of the observable universe a sphere around 92 billion light-years. 

These estimations are further complicated by the possibility that the universe is not expanding in an even manner. ESA reported on a 2020 study using data from ESA’s XMM-Newton, NASA’s Chandra Space Telescope and Rosat X-ray observatories suggests that the universe is not expanding at the same rate in all directions. The study measured the X-ray temperatures of hundreds of galaxy clusters and compared that against their brightness. Some clusters appeared less bright than expected, suggesting they were not moving at the same rate. "This possibly uneven effect on cosmic expansion might be caused by the mysterious dark energy ," ESA stated.

Centering a sphere on Earth's location in space might seem to put humans in the center of the universe. However, like that same ship in the ocean, we cannot tell where we lie in the enormous span of the universe. Just because we cannot see land does not mean we are in the center of the ocean; just because we cannot see the edge of the universe does not mean we lie in the center of the universe.

Scientists measure the size of the universe in a myriad of different ways. They can measure the waves from the early universe, known as baryonic acoustic oscillations, that fill the cosmic microwave background. They can also use standard candles, such as type 1A supernovae, to measure distances. However, these different methods of measuring distances can provide answers.

How inflation is changing is also a mystery. While the estimate of 92 billion light-years comes from the idea of a constant rate of inflation, many scientists think that the rate is slowing down. If the universe expanded at the speed of light during inflation, it should be 10^23, or 100 sextillion. One explanation for this, outlined by NASA in 2019, is that dark energy events may have impacted the expansion of the universe in the moments after the Big Bang.

Instead of taking one measurement method, a team of scientists led by Mihran Vardanyan at the University of Oxford did a statistical analysis of all of the results. By using Bayesian model averaging, which focuses on how likely a model is to be correct given the data, rather than asking how well the model itself fits the data. They found that the universe is at least 250 times larger than the observable universe, or at least 7 trillion light-years across.

"That's big, but actually more tightly constrained that many other models," according to 2011 MIT Technology Review report.

Related stories:

– Phantom energy and dark gravity: Explaining the dark side of the universe

– Do parallel universes exist? We might live in a multiverse

– Geocentric model: The Earth-centered view of the universe

The size of the universe depends a great deal on its shape. Scientists have predicted the possibility that the universe might be closed like a sphere, infinite and negatively curved like a saddle, or flat and infinite.

A finite universe has a finite size that can be measured; this would be the case in a closed spherical universe. But an infinite universe has no size by definition.

According to NASA, scientists know that the universe is flat with only about a 0.4 percent margin of error (as of 2013). And that could change our understanding of just how big the universe is.

"This suggests that the universe is infinite in extent; however, since the universe has a finite age, we can only observe a finite volume of the universe," says NASA . "All we can truly conclude is that the universe is much larger than the volume we can directly observe."

Determining the shape of the universe presents further challenges thanks to the limits of our means of observation. "Like a hall of mirrors, the apparently endless universe might be deluding us. The cosmos could, in fact, be finite. The illusion of infinity would come about as light wrapped all the way around space, perhaps more than once—creating multiple images of each galaxy," according to the University of Oregon department of physics . 

There are plenty more questions about the universe you might want to have answered, such as what if the universe didn't have a beginning ? If your thirst for universal knowledge needs more, then these 10 wild theories about the universe might get your mind racing as well.

"Planck Mission Explores the History of Our Universe" NASA Jet Propulsion Laboratory

"How Big Was The Universe At The Moment Of Its Creation?" Forbes

"The Universe Might Not Be Expanding At The Same Rate Everywhere" ESA

"Mystery of the Universe's Expansion Rate Widens With New Hubble Data" NASA

"Cosmos At Least 250x Bigger Than Visible Universe, Say Cosmologists" MIT Technology Review

"The Universe Is Flat — Now What?" Space.com

"Will the Universe expand forever?" NASA

"Geometry of the Universe" University of Oregon department of physics

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Nola Taylor Tillman is a contributing writer for Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd

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Age & Size of the Universe Through the Years

Our estimate of the age and size of the Universe has changed during the past century. Cosmic Times reflects this through the "Age of the Universe" and "Size of the Universe" boxes in the upper left and upper right of each edition. The Size refers to the diameter of the known universe.

Here we describe these estimates.

1919 Age: Infinite Size: 300,000 Light Years

In the early 20th century, astronomers thought that the Universe was infinitely old and unchanging. Meanwhile geologists were determining the age of the earth to be about 1.6 billion years old using early applications of radioactive decay.

At the time, some astronomers thought that the Milky Way comprised everything in the Universe. As described in the article Mt. Wilson Astronomer Estimates Milky Way Ten Times Bigger than Thought , Harlow Shapley studied the distances to globular clusters to determine the size of the Milky Way Galaxy to be 300,000 light years across.

1929 Age: 2 Billion Years Size: 280 Million Light Years

In 1924, Edwin Hubble determined the distance to the Andromeda Nebula to be 900,000 light years. By 1929, he had measured the distances to 24 additional spiral nebulae in his study to determine distances to the galaxies for which Slipher had previously determined redshifts. The farthest was 140 million light years away, making the universe 280 million light years across.

One result from Hubble's discovery of the relationship between the recessional velocity and distance to distant galaxies is that the constant which defines that relationship is also related to the age of the universe. If the universe has been expanding, and Hubble's constant gives the expansion rate, then its inverse gives the amount of time that the expansion has been going on. Hubble's initial value of this constant gives an age of the universe of 2 billion years. Interestingly, at this same time geologists had determined the age of the Earth to be 3 billion years.

1955 Age: 6 Billion Years Size: 4 Billion Light Years

As a result of the recalibration of the Cepheid distance scale and of the new results from the 200-inch telescope at Mt. Palomar, the size of the Universe increased to 4 billion light years by the mid-1950's.

In 1952, Walter Baade redetermined the value of Hubble's constant to be much lower than what Hubble had estimated. As a result, the Universe was found to be about 6 billion years old.

1965 Age: 10-25 Billion Years Size: 25 Billion Light Years

The farthest objects in 1965 were the quasars. The most distant known quasar, named 3C9, was found to be about 12 billion light years away. This gives a size for the universe of about 25 billion light years.

In 1958, Alan Sandage again lowered the value of Hubble's constant, but ended up with a range of ages for the Universe between 15 and 25 billion years. As of 1965, this uncertainty remained, since subsequent studies by a variety of astronomers found different values within this range.

1993 Age: 12-20 Billion Years Size: 30 Billion Light Years

Quasars continue to define the size of the universe into the early 1990's. Quasars had been found with recessional velocities nearly 90% the speed of light, giving distances of 15 billion light years. This gives a size of the universe of 30 billion light years across.

The value of Hubble's constant remained uncertain, giving a range in age for the universe of 12-20 billion years.

2006 Age: 13.7 Billion Years Size: 94 Billion Light Years

The most distant objects in the Universe are 47 billion light years away, making the size of the observable Universe 94 billion light years across. How can the observable universe be larger than the time it takes light to travel over the age of the Universe? This is because the universe has been expanding during this time. This causes very distant objects to be further away from us than their light travel time. For additional information, see Ned Wright's Cosmology FAQ .

The Hubble Key Project, conducted by the Hubble Space Telescope from 1991 to 2000, nailed down the value of the Hubble Constant and hence the age of the Universe. Results from the WMAP satellite further confirmed and refined the age of the Universe to be 13.7 billion years.

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How Big is the Universe?

Think about this for a second; it takes us around three days to reach the Moon , approximately seven months to get the closest planet to us, namely Mars , 15 months to reach Venus , six years to reach Jupiter , seven to reach Saturn , 8.5 years to reach Uranus , 9.5 years to reach Pluto – the closest dwarf planet , and twelve years to get to Neptune , the farthest planet.

The Sun is 0.00001581 light-years away, and in the best-case scenario, we could reach it in 25 days. So how big is the Universe ? Its around 93 billion light-years. How much is that? Well, let’s think about the Sun again.

The Sun is one astronomical unit (AU) away from us. One astronomical unit is 149,598,000 km / 92,955,887 miles, and in our top shape, we could reach it in 25 days. Now, the Universe is 93 billion light-years across, and one, just one light-year, is equivalent to 63,000 astronomical units. 

As such, one light-year is the equivalent to 9 trillion kilometers / 6 trillion miles, and our Universe is 93 billion light-years in diameter. That’s how big our Universe is, and that’s not even the end of it. The 93 billion years is just the observable Universe, the Universe, which we can currently see. The whole Universe might very well be 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.

Why is the Universe so Big?

The Universe is so big because it is constantly expanding, and it does so at a speed that even exceeds the speed of light. Space itself is actually growing, and this is going on for around 14 billion years or so.

In this amount of time, with speed greater than the speed of light, the Universe gradually grew, and it still expands even to this day. There isn’t actually an answer to why the Universe is this big.

Think about how big you are compared to an ant or an atom. There is a huge difference even there. It all has to do with our perception, and even in our current modern age, our perception of the Universe is vastly limited.

Think of it this way, we might see the Universe as being big now, but in the far future, who knows how future generations would view it. Our ancestors didn’t have cars or planes, and they would traverse the world in many months, even years. For them, traveling from point a to b seemed incredibly difficult, and they themselves might have thought, why is the Earth so big?

Nowadays, the world doesn’t seem so big anymore. You can reach point b from point a in a couple of minutes, hours, or in the worst case, days. Our perception is the only thing at play here. 

In the future, who knows, maybe we could actually invent something that could travel at the speed of light. Perhaps we might even invent teleportation, or we could use wormholes.

If such technologies are there for us to grab, then will the Universe truly seem so big then? Perhaps it will, but if the vast distances which we should traverse are more easily done, and if the man of the future will find a way to prolong his life further, then the Universe will certainly begin to feel much smaller. Again, perception is everything at play here. We are asking the wrong questions; nature is what it is. 

Is the Universe Really Infinite?

Many believe that our Universe is just 13.8 billion years old. However, this is uncertain until proven with extreme accuracy. Sometimes, we can’t even pinpoint the certain age of an object here on Earth, let alone our Universe.

The Universe may be infinite, or it might not be, but again, our perception is at play here. If we will analyze how many stars , planets, and the distances involved for reaching them, and the fact that our Universe is expanding, then it certainly seems infinite.

Even if our Universe weren’t infinite, we would view it as such due to its vastness, and the time it would take to explore everything in it. Our perception creates endless problems in the end.

Now, as you observe, our world is created from dualistic elements—day and night, hot or cold, love or hate, etc. Everything in our little world seems finite, so why shouldn’t there be an infinite element, such as our Universe? Many are afraid to admit that something is infinite, but no matter how you look at it, even if you would reach the end of the Universe, it would still seem infinite regardless.

How Big is the Universe Compared to the Observable Universe?

The observable Universe is 93 billion light-years in diameter. Some scientists believe its true size is even scarier than that. By using the Bayesian model averaging, scientists estimated that the Universe is at least 250 times larger than the observable Universe, or at least 7 trillion light-years in diameter.

The Bayesian model focuses on how likely a model is to be correct, given the data, rather than asking how well the model itself fits the data. Now, this might not be the best method of estimating the true size of our Universe. Still, chances are very high that our Universe is nonetheless, bigger than the observable Universe.

Does the Universe End?

Many cosmologists agree that the Universe is flat and might expand forever. Others theorize that the Universe will end one day. Perhaps the most logical answer is that when all the stars reach the end of their life spans, the Universe will end as the light will end, everything will be covered in darkness.

Maybe the Universe won’t end there if other means of survival are available. Some believe that the Universe will begin to cool as it expands, and life will cease to exist because of this. This theory is named the Big Freeze.

Another popular scenario suggests that our Universe will stop expanding, and will actually reverse this process. When this happens, the Universe will re-collapse, and it will possibly lead to a reformation that will start with another Big Bang. This scenario is called the Big Crunch.

A third popular theory is named the Big Rip. This theory states that everything will be torn to shreds, including atoms. It will happen when the theoretical energy known as dark energy, becomes stronger than gravity. No matter the case, what is certain is that these apocalyptic scenarios won’t happen for billions of years to come.

Where Does Space End?

Space doesn’t end because it is constantly expanding, faster than the speed of light itself. From our point of view, space end at approximately 93 billion light-years away from us, but that isn’t the end of the Universe. It is only how far we can see.

Will the Universe Last Forever?

It is unknown if the Universe will last forever, but most likely, we won’t even be there to see it. We currently do not know if the Univers will stop expanding, and if it does, what would this imply.

Many have proposed several apocalyptic scenarios, like the ones aforementioned above, but isn’t that just typical of us? The Universe might just well last forever, but one thing is true. We are very far from answering such questions.

Everything in the Universe is in motion, and it appears that many celestial objects, such as galaxies , are moving away from us. Perhaps, this is the true end of the Universe, when things will be so far apart that nothing could be reached anymore, and nothing could be concluded as being the Universe anymore since everything will be so far apart, we wouldn’t even know its there.

Did you Know?

• Around 550 people have been into Space, and only three of them have died in accidents.
• The smallest thing in the Universe that we currently know of is the atom.
• The biggest thing we have discovered so far in our Universe is the Hercules-Corona Borealis Great Wall. It is a supercluster which has a diameter of around 10 billion light-years.
• Many believe that our Universe is only one of a set of separate universes, collectively known as the multiverse.
• The word cosmos, rather than Universe, implies viewing the Universe as a complex and orderly system or deity – the opposite of chaos.
• The observable Universe is 93 billion light-years, yet, our galaxy, the Milky Way , is just 100,000 light-years in diameter. It would take us endless generations just to explore our galaxy, let alone the Universe.
• Another ancient structure is the galaxy supercluster known as the Hyperion Supercluster. This celestial object has more than four quadrillion solar masses, and many estimate that it formed just 2 billion years after the Big Bang.
• Universe means “whole,” and it comes from the Latin word “universus.”
• Livescience
• Scaleofuniverse

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How big is the universe? The shape of space-time could tell us

We may never know what lies beyond the boundaries of the observable universe, but the fabric of the cosmos can tell us whether the universe is infinite or not

By Leah Crane

25 June 2024

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This story is part of our Cosmic Perspective series, in which we confront the staggering vastness of the cosmos and our place in it. Read the rest of the series  here .

In a sense, we are at the centre of the universe – but only because we can see the same distance in every direction, giving us the perfectly spherical observable universe. The speed limit of light combined with the inexorable expansion of the cosmos means that we can see about 46 billion light years in every direction. What lies beyond this horizon? That is a mystery we may never solve.

But there are clues. Two competing effects govern the overall size of the universe: gravity and dark energy . All matter has mass, which causes gravitational forces that pull everything towards everything else. To their surprise, however, cosmologists in the early 20th century found that distant galaxies seem to be hurtling away from us. The mysterious force causing this strange expansion of space was dubbed dark energy, and its nature remains elusive to this day.

Have we been measuring the expansion of the universe wrong all along?

“Up until the discovery of dark energy and the acceleration of expansion, the universe was simpler,” says cosmologist Wendy Freedman at the University of Chicago. Without dark energy, the universe would be much smaller and its size easier to predict.

Even with dark energy, the universe may only be slightly larger than what is observable. In March, Jean-Luc Lehners , then at the Max Planck Institute for Gravitational Physics in Germany, and Jerome Quintin at the University of Waterloo in Canada …

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Size and Scale of the Universe

Jan 01, 2020

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Size and Scale of the Universe. Image courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002. Earth. Planet where we all live Comprised primarily of rock Spherical in shape 12,700 km in diameter

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Presentation Transcript

Size and Scale of the Universe Image courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002

Earth • Planet where we all live • Comprised primarily of rock • Spherical in shape • 12,700 km in diameter • It would take 17 days to circumnavigate the globe driving a car at 100 km/hr • At the speed of light, it would take 0.13 seconds to go all the way around Earth.

Sun • Star that Earth orbits • Composed primarily of hydrogen and helium gas • Uses nuclear fusion in its core to generate heat and light to allow itself to resist the crushing weight of its own mass • Spherical in shape • 1.39 Million km in diameter

Earth & Sun • The Sun’s diameter is 109 times greater than that of Earth • Over 1 million Earths would fit inside the Sun’s volume • Earth orbits the Sun at an average distance of 150 million kilometers. This distance is called an Astronomical Unit (AU) • It would take 11,780 Earths lined up side to side to bridge the 1 AU between Earth and Sun.

The Solar System • 8.5 planets, thousands and thousands of planetoids and asteroids, billions of comets and meteoroids • Mostly distributed in a disk about the Sun • Sun blows a constant wind of charged gas into interplanetary space, called the Solar Wind Boundary between Solar Wind and interstellar space at 100 AU from the Sun (200 AU diameter)

The Solar Neighborhood • The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood. • Here stars move generally with the Sun in its orbit around the center of the Galaxy • This region is inside a large bubble of hot interstellar gas called the Local Bubble. Here the gas temperature is about 1 million degrees Kelvin and the density is 1000 times less than average interstellar space. To Center of Galaxy The image is 390 light-years across. Direction of Galactic Rotation

You Are Here The Milky Way Galaxy The Milky Way Galaxy is a giant disk of stars 160,000 light-years across and 1,000 light-years thick. The Sun is located at the edge of a spiral arm, 30,000 light-years from the center It takes 250 Million years for the Sun to complete one orbit There are over 100 Billion stars in the Milky Way The Spiral arms are only 5% more dense than average, and are the locations of new star formation

The Local Group • Contains 3 large spiral galaxies--Milky Way, Andromeda (M31), and Triangulum (M33)—plus a few dozen dwarf galaxies with elliptical or irregular shapes. • Gravitationally bound together—orbiting about a common center of mass • Ellipsoidal in shape • About 6.5 million light-years in diameter

A cluster of many groups and clusters of galaxies • Largest cluster is the Virgo cluster containing over a thousand galaxies. • Clusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expands. • The Local Supercluster gets bigger with time • It has a flattened shape • The Local Group is on the edge of the majority of galaxies • The Local Supercluster is about 130 Million light-years across The Local Supercluster

1.3 Billion light-years The Universe • Surveys of galaxies reveal a web-like or honeycomb structure to the Universe • Great walls and filaments of matter surrounding voids containing no galaxies • Probably 100 Billion galaxies in the Universe The plane of the Milky Way Galaxy obscures our view of what lies beyond. This creates the wedge-shaped gaps in all-sky galaxy surveys such as those shown here.

The Universe The observable Universe is 27 Billion light-years in diameter. Computer Simulation

1) The Standard Ruler There are two basic methods for measuring astronomical distances • Use knowledge of physical and/or geometric properties of an object to relate an angular size with a physical size to determine distance. • Ex: Parallax, Moving Clusters, Time Delays, Water MASERs • Considered to be a direct or absolute measurement. R  d d = R/Tan()  R/

Requires very precise measurements of stellar positions, and long baselines Need telescopes with high resolution, and must observe over several years. Hipparchos satellite measured distances to tens of thousands of stars within 1,500 light-years of the Sun. Trigonometric Parallax

Use knowledge of physical and/or empirical properties of an object to determine its Luminosity, which yields distance via the Inverse Square Law of Light. Ex: Cepheid Variables, Supernovae, TRGB, Tully-Fisher Considered to be relative until tied to an absolute calibration. 2) The Standard Candle b = L/4d2

Cepheid Variable Stars There is a kind of giant star whose surface pulsates in and out with a regular period. That period of pulsation is related to the Luminosity of the star. LMC contains hundreds of known Cepheids all at the same distance. Which allows for robust determination of the Period Luminosity Relationship.

To measure cosmological distances a ladder of methods is used to reach further out into the Universe. Each “rung” in the ladder of distance measuring methods depends on the calibration of the methods “below.”

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• Preferences

Size and Scale of the Universe - PowerPoint PPT Presentation

Size and Scale of the Universe

The region of the galaxy within about 32.6 light-years of the sun (65 light ... the milky way galaxy is a giant disk of stars 160,000 light-years across and 1, ... – powerpoint ppt presentation.

• Planet where we all live
• Comprised primarily of rock
• Spherical in shape
• 12,700 km in diameter
• It would take 17 days to circumnavigate the globe driving a car at 100 km/hr
• At the speed of light, it would take 0.13 seconds to go all the way around Earth.
• Star that Earth orbits
• Composed primarily of hydrogen and helium gas
• Uses nuclear fusion in its core to generate heat and light to allow itself to resist the crushing weight of its own mass
• 1.39 Million km in diameter
• The Suns diameter is 109 times greater than that of Earth
• Over 1 million Earths would fit inside the Suns volume
• Earth orbits the Sun at an average distance of 150 million kilometers. This distance is called an Astronomical Unit (AU)
• It would take 11,780 Earths lined up side to side to bridge the 1 AU between Earth and Sun.
• 8.5 planets, thousands and thousands of planetoids and asteroids, billions of comets and meteoroids
• Mostly distributed in a disk about the Sun
• Sun blows a constant wind of charged gas into interplanetary space, called the Solar Wind
• The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood.
• Here stars move generally with the Sun in its orbit around the center of the Galaxy
• This region is inside a large bubble of hot interstellar gas called the Local Bubble. Here the gas temperature is about 1 million degrees Kelvin and the density is 1000 times less than average interstellar space.
• Contains 3 large spiral galaxies--Milky Way, Andromeda (M31), and Triangulum (M33)plus a few dozen dwarf galaxies with elliptical or irregular shapes.
• Gravitationally bound togetherorbiting about a common center of mass
• Ellipsoidal in shape
• About 6.5 million light-years in diameter
• A cluster of many groups and clusters of galaxies
• Largest cluster is the Virgo cluster containing over a thousand galaxies.
• Clusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expands.
• The Local Supercluster gets bigger with time
• It has a flattened shape
• The Local Group is on the edge of the majority of galaxies
• The Local Supercluster is about 130 Million light-years across
• Surveys of galaxies reveal a web-like or honeycomb structure to the Universe
• Great walls and filaments of matter surrounding voids containing no galaxies
• Probably 100 Billion galaxies in the Universe
• Use knowledge of physical and/or geometric properties of an object to relate an angular size with a physical size to determine distance.
• Ex Parallax, Moving Clusters, Time Delays, Water MASERs
• Considered to be a direct or absolute measurement.
• Requires very precise measurements of stellar positions, and long baselines
• Need telescopes with high resolution, and must observe over several years.
• Hipparchos satellite measured distances to tens of thousands of stars within 1,500 light-years of the Sun.
• Use knowledge of physical and/or empirical properties of an object to determine its Luminosity, which yields distance via the Inverse Square Law of Light.
• Ex Cepheid Variables, Supernovae, TRGB, Tully-Fisher
• Considered to be relative until tied to an absolute calibration.
• There is a kind of giant star whose surface pulsates in and out with a regular period. That period of pulsation is related to the Luminosity of the star.
• LMC contains hundreds of known Cepheids all at the same distance. Which allows for robust determination of the Period Luminosity Relationship.

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Astronomy Picture of the Day

What is the Universe?

The universe is everything. It includes all of space, and all the matter and energy that space contains. It even includes time itself and, of course, it includes you.

Earth and the Moon are part of the universe, as are the other planets and their many dozens of moons. Along with asteroids and comets, the planets orbit the Sun. The Sun is one among hundreds of billions of stars in the Milky Way galaxy, and most of those stars have their own planets, known as exoplanets.

The Milky Way is but one of billions of galaxies in the observable universe — all of them, including our own, are thought to have supermassive black holes at their centers. All the stars in all the galaxies and all the other stuff that astronomers can’t even observe are all part of the universe. It is, simply, everything.

Though the universe may seem a strange place, it is not a distant one. Wherever you are right now, outer space is only 62 miles (100 kilometers) away. Day or night, whether you’re indoors or outdoors, asleep, eating lunch or dozing off in class, outer space is just a few dozen miles above your head. It’s below you too. About 8,000 miles (12,800 kilometers) below your feet — on the opposite side of Earth — lurks the unforgiving vacuum and radiation of outer space.

In fact, you’re technically in space right now. Humans say “out in space” as if it’s there and we’re here, as if Earth is separate from the rest of the universe. But Earth is a planet, and it’s in space and part of the universe just like the other planets. It just so happens that things live here and the environment near the surface of this particular planet is hospitable for life as we know it. Earth is a tiny, fragile exception in the cosmos. For humans and the other things living on our planet, practically the entire cosmos is a hostile and merciless environment.

How old is Earth?

Our planet, Earth, is an oasis not only in space, but in time. It may feel permanent, but the entire planet is a fleeting thing in the lifespan of the universe. For nearly two-thirds of the time since the universe began, Earth did not even exist. Nor will it last forever in its current state. Several billion years from now, the Sun will expand, swallowing Mercury and Venus, and filling Earth’s sky. It might even expand large enough to swallow Earth itself. It’s difficult to be certain. After all, humans have only just begun deciphering the cosmos.

While the distant future is difficult to accurately predict, the distant past is slightly less so. By studying the radioactive decay of isotopes on Earth and in asteroids, scientists have learned that our planet and the solar system formed around 4.6 billion years ago.

How old is the universe?

The universe, on the other hand, appears to be about 13.8 billion years old. Scientists arrived at that number by measuring the ages of the oldest stars and the rate at which the universe expands. They also measured the expansion by observing the Doppler shift in light from galaxies, almost all of which are traveling away from us and from each other. The farther the galaxies are, the faster they’re traveling away. One might expect gravity to slow the galaxies’ motion from one another, but instead they’re speeding up and scientists don’t know why. In the distant future, the galaxies will be so far away that their light will not be visible from Earth.

Put another way, the matter, energy and everything in the universe (including space itself) was more compact last Saturday than it is today.

Put another way, the matter, energy and everything in the universe (including space itself) was more compact last Saturday than it is today. The same can be said about any time in the past — last year, a million years ago, a billion years ago. But the past doesn’t go on forever.

By measuring the speed of galaxies and their distances from us, scientists have found that if we could go back far enough, before galaxies formed or stars began fusing hydrogen into helium, things were so close together and hot that atoms couldn’t form and photons had nowhere to go. A bit farther back in time, everything was in the same spot. Or really the entire universe (not just the matter in it) was one spot.

Don't spend too much time considering a mission to visit the spot where the universe was born, though, as a person cannot visit the place where the Big Bang happened. It's not that the universe was a dark, empty space and an explosion happened in it from which all matter sprang forth. The universe didn’t exist. Space didn’t exist. Time is part of the universe and so it didn’t exist. Time, too, began with the big bang. Space itself expanded from a single point to the enormous cosmos as the universe expanded over time.

What is the universe made of?

The universe contains all the energy and matter there is. Much of the observable matter in the universe takes the form of individual atoms of hydrogen, which is the simplest atomic element, made of only a proton and an electron (if the atom also contains a neutron, it is instead called deuterium). Two or more atoms sharing electrons is a molecule. Many trillions of atoms together is a dust particle. Smoosh a few tons of carbon, silica, oxygen, ice, and some metals together, and you have an asteroid. Or collect 333,000 Earth masses of hydrogen and helium together, and you have a Sun-like star.

For the sake of practicality, humans categorize clumps of matter based on their attributes. Galaxies, star clusters, planets, dwarf planets, rogue planets, moons, rings, ringlets, comets, meteorites, raccoons — they’re all collections of matter exhibiting characteristics different from one another but obeying the same natural laws.

Scientists have begun tallying those clumps of matter and the resulting numbers are pretty wild. Our home galaxy, the Milky Way, contains at least 100 billion stars, and the observable universe contains at least 100 billion galaxies. If galaxies were all the same size, that would give us 10 thousand billion billion (or 10 sextillion) stars in the observable universe.

But the universe also seems to contain a bunch of matter and energy that we can’t see or directly observe. All the stars, planets, comets, sea otters, black holes and dung beetles together represent less than 5 percent of the stuff in the universe. About 27 percent of the remainder is dark matter, and 68 percent is dark energy, neither of which are even remotely understood. The universe as we understand it wouldn’t work if dark matter and dark energy didn’t exist, and they’re labeled “dark” because scientists can’t seem to directly observe them. At least not yet.

How has our view of the universe changed over time?

Human understanding of what the universe is, how it works and how vast it is has changed over the ages. For countless lifetimes, humans had little or no means of understanding the universe. Our distant ancestors instead relied upon myth to explain the origins of everything. Because our ancestors themselves invented them, the myths reflect human concerns, hopes, aspirations or fears rather than the nature of reality.

Several centuries ago, however, humans began to apply mathematics, writing and new investigative principles to the search for knowledge. Those principles were refined over time, as were scientific tools, eventually revealing hints about the nature of the universe. Only a few hundred years ago, when people began systematically investigating the nature of things, the word “scientist” didn’t even exist (researchers were instead called “natural philosophers” for a time). Since then, our knowledge of the universe has repeatedly leapt forward. It was only about a century ago that astronomers first observed galaxies beyond our own, and only a half-century has passed since humans first began sending spacecraft to other worlds.

In the span of a single human lifetime, space probes have voyaged to the outer solar system and sent back the first up-close images of the four giant outermost planets and their countless moons; rovers wheeled along the surface on Mars for the first time; humans constructed a permanently crewed, Earth-orbiting space station; and the first large space telescopes delivered jaw-dropping views of more distant parts of the cosmos than ever before. In the early 21st century alone, astronomers discovered thousands of planets around other stars, detected gravitational waves for the first time and produced the first image of a black hole.

With ever-advancing technology and knowledge, and no shortage of imagination, humans continue to lay bare the secrets of the cosmos. New insights and inspired notions aid in this pursuit, and also spring from it. We have yet to send a space probe to even the nearest of the billions upon billions of other stars in the galaxy. Humans haven’t even explored all the worlds in our own solar system. In short, most of the universe that can be known remains unknown .

The universe is nearly 14 billion years old, our solar system is 4.6 billion years old, life on Earth has existed for maybe 3.8 billion years, and humans have been around for only a few hundred thousand years. In other words, the universe has existed roughly 56,000 times longer than our species has. By that measure, almost everything that’s ever happened did so before humans existed. So of course we have loads of questions — in a cosmic sense, we just got here.

Our first few decades of exploring our own solar system are merely a beginning. From here, just one human lifetime from now, our understanding of the universe and our place in it will have undoubtedly grown and evolved in ways we can today only imagine.

Next: The Search for Life: Are We Alone?

• The Universe

The Universe is everything we can touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space and matter did not exist.

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays).

The Universe is incredibly huge. It would take a modern jet fighter more than a million years to reach the nearest star to the Sun. Travelling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone.

No one knows the exact size of the Universe, because we cannot see the edge – if there is one. All we do know is that the visible Universe is at least 93 billion light years across. (A light year is the distance light travels in one year – about 9 trillion km.)

The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart as the space between them expands.

Story of the Universe

• Extreme life
• In the beginning
• The Big Bang
• The birth of galaxies
• What is space?
• Black Holes
• The mystery of the dark Universe
• Cosmic distances