Download Article: How Big is our Universe

How Big is Our Universe?
07.15.04
The universe is a big, big place. But how big? And how do we know?
Throughout history, humans have used a variety of techniques and methods to help them answer the
questions 'How far?' and 'How big?' Generations of explorers have looked deeper and deeper into the vast
expanse of the universe. And the journey continues today, as new methods are used, and new discoveries
are made. (To learn more about distance, visit How Big is Our Universe.)
In the third century B.C., Aristarchus of Samos asked the question 'How far away is the Moon?' He was able
to measure the distance by looking at the shadow of the Earth on the Moon during a lunar eclipse.
It was Edmund Halley, famous for predicting the return of the comet that bears his name, who three
centuries ago found a way to measure the distance to the Sun and to the planet Venus. He knew that the
planet Venus would very rarely, every 121 years, pass directly between the Earth and the Sun. The
apparent position of the planet, relative to the disk of the Sun behind it, is shifted depending on where you
are on Earth. And how different that shift is depends on the distance from both Venus and the Sun to the
Earth. This rare event, the transit of Venus, occurred again quite recently, June 8, 2004.
It was knowing this fundamental distance from the Earth to the Sun that helped us find the true scale of the
entire Solar system for the first time.
Image to right: Our sun, the nearest star,
is 93 million miles away. That's why the
sun, which is a million times the size of
the Earth, looks so small. It would take the
Space Shuttle seven months to fly there.
Credit: SOHO - ESA & NASA
When we leave the solar system, we find
our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of
stars, so big that even at the speed of light, it would take 100,000 years to travel across it. All the stars in the
night sky, including our Sun, are just some of the residents of this galaxy, along with millions of other stars
too faint to be seen.
The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to
stars that are very far away. But how do you know if the star really is far away, or just not very bright to begin
with? This problem was solved in 1908 when Henrietta Leavitt discovered a way to tell the 'wattage' of
certain stars that changed their pulse rate linked to their wattage. This allowed their distances to be
measured all the way across the Milky Way.
Image above: How Big is the Milky Way? Imagine that our entire
Solar System were the size of a quarter. The Sun is now a
microscopic speck of dust, as are its nine planets, whose orbits are
represented by the flat disc of the coin. How far away is the nearest
star to our sun? In our model, Proxima Centauri (and any planets
that might be around it) would be another quarter, two soccer fields
away. This is the typical separation of stars in our part of the galaxy.
Credit: Hubble Heritage Team (AURA/STSCI/NASA); US Mint
Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies
we discover. There are billions of galaxies, the most distant of which are so far away that the light arriving
from them on Earth today set out from the galaxies billions of years ago. So we see them not as they are
today, but as they looked long before there was any life on Earth.
Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for
incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness
- wattage - so we can figure out how far they are by measuring how bright they appear to us, and therefore
how far away it is to their home galaxy.
Image to right: The picture on the right was taken three weeks
after the one on the left. In that time, a star at the edge of one of
these distant galaxies has exploded -- "gone supernova." Can you
spot the supernova in the picture at right? Even though the
explosion is as bright as a billion suns, it is so far away that it is
just a speck of light. Credit: NASA and J. Blakeslee (JHU)
The image below is both the oldest and youngest picture ever
taken. It is the oldest because it has taken the light nearly 14
billion years to reach us. And it is the youngest because it is a snapshot of our newborn universe, long
before the first stars and galaxies formed. The bright patterns show clumps of simple matter that will
eventually form stars and galaxies. This is as far as we can see into the universe. It is time, not space, which
limits our view. Beyond a certain distance, light hasn't had time to reach us yet.
Image above: What is the furthest we can see? In 2003, NASA's
WMAP satellite took images of the most distant part of the universe
observable from Earth. The image shows the furthest we can see
using any form of light. The patterns show clumps of matter that
eventually formed into galaxies of stars. Credit: NASA/WMAP
Science Team
So how big is the universe? No one knows if the universe is infinitely large, or even if ours is the only
universe that exists. And other parts of the universe, very far away, might be quite different from the
universe closer to home. Future NASA missions will continue to search for clues to the ultimate size and
scale of our cosmic home.
http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html
Name__________________________________Date______________Period_____
Questions
“How Big is Our Universe?”
1. How did Aristarchus of Samos measure how far away the moon is?
2. When was the last time that Venus passed directly in between the Earth and the sun?
3. What is the phenomenon in number 2 used to measure the size of?
4. Why does the sun look so small even though it is a million times the size of Earth?
5. How long would it take to travel across the Milky Way even if you were going the speed of light?
6. If our entire solar system was the size of a quarter, how large would the nearest star be, and how far away?
7. How many galaxies do scientists believe are out there?
8. What do scientists watch for to measure distances to far off galaxies?
9. Do we know how big the Universe is for sure?
Name________________________________________Date____________Period_____
Questions
“How Big is Our Universe?”
1. How did Aristarchus of Samos measure how far away the moon is?
2. When was the last time that Venus passed directly in between the Earth and the sun?
3. What is the phenomenon in number 2 used to measure the size of?
4. Why does the sun look so small even though it is a million times the size of Earth?
5. How long would it take to travel across the Milky Way even if you were going the speed of light?
6. If our entire solar system was the size of a quarter, how large would the nearest star be, and how far away?
7. How many galaxies do scientists believe are out there?
8. What do scientists watch for to measure distances to far off galaxies?
9. Do we know how big the Universe is for sure?