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Home , Andromeda Galaxy, Small Magellanic Cloud, Virgo (constellation)

Stars and

St u d e n t Pa g e Se e i n g i n to t h e Pa s t A is a gravitationally bound system of , gas, and dust. Gal- We can’t travel into the past, but we axies range in diameter from a few thousand to a few hundred thou- can get a glimpse of it. Every sand light-years. Each galaxy contains billions (10 9) or trillions (1012) time we look at the Moon, for of stars. In this activity, you will apply concepts of scale to grasp the example, we see it as it was a distances between stars and galaxies. You will use this understanding little more than a second ago. to elaborate on the question, Do galaxies collide? That’s because sunlight reflected from the Moon’s surface takes a little more Ex p l o r e than a second to reach . We see On a clear, dark night, you can see hundreds of bright stars. The next table the Sun as it looked about eight minutes shows some of the brightest stars with their diameters and distances from ago, and the other stars as they were a the Sun. Use a calculator to determine the scaled distance to each few years to a few centuries ago. (how many times you could fit the star between itself and the Sun). Hint: And then there’s M31, the Androm- you first need to convert light-years and solar diameters into meters. One eda galaxy — the most distant object light-year equals 9.46 x 1015 meters, and the Sun’s diameter is 1.4 x 109 that’s readily visible to human eyes. meters. This great amalgamation of stars stands almost directly overhead late this eve- Star Diameter Distance Scaled Distance ning. When viewed from a dark sky- () (Sun=1) (light-years) (distance÷diameter) watching location, far from city lights, () 8 261 it looks like a faint, fuzzy blob. But that blob is the combined glow of hundreds Betelgeuse () 600 489 of billions of stars — seen as it looked Deneb () 200 1,402 more than two million years ago. Altair () 2 17 is like a larger version of Vega () 2.7 26 our own galaxy. It’s a flat disk that spans more than a quarter-mil- () 1.6 8.6 lion light-years. Its brightest stars form spiral arms that make the galaxy look There are three galaxies beyond the Milky Way that you can see without like a pinwheel. Yet the galaxy is so optical aid: the , the , and the far away that its structure is visible only . Figure the scaled distance to these galaxies (how through telescopes. many times you could fit the galaxy between itself and the Milky Way). The light from M31 has to travel about two and a half million light-years Diameter Distance Scaled Distance Galaxy (distance÷diameter) to reach us — about 15 quintillion (light-years) (light-years) miles — the number 15 followed by (no conversion needed) 18 zeroes. Yet even across such an Milky Way 100,000 0 enormous gulf, the galaxy is so bright Andromeda Galaxy 125,000 2,500,000 that we can see it — faintly — with our own eyes, crossing high overhead late Large Magellanic Cloud 31,000 165,000 tonight. Small Magellanic Cloud 16,000 200,000

Ex p l a i n How does the scaled distance of galaxies compare to stars? This is the transcript of a StarDate radio episode that aired October 14, 2006. Script by Damond Benningfield, ©2006.

El a b o r at e Do you think galaxies collide? Why or why not?

28 S t a r D a t e /Un i v e r s o Te a c h e r Gu i d e Texas Essential Knowledge and Skills

Science: §112.20. grade 8 - 8(D) model and describe how light years are used to measure distances and sizes in the . , grade 9-12: §112.33.(c) - 6(C) examine the scale, size, and distance of the stars, Milky Way, and other galaxies through the use of data and modeling. §112.33.(c) - 6(E) demonstrate the use of units of measurement in astronomy, including Astronomical Units and light years.