Astronomy 162: Professor Barbara Ryden

Monday, February 17

WHAT IS A GALAXY?

(THE LECTURE THAT WASN'T!!)


``See yonder, lo, the Galaxye,
Which men clepeth the Milky Wey,
For hit is whyt.''
- Geoffrey Chaucer, The House of Fame

Key Concepts


(1) A galaxy is a large assembly of stars, gas and dust, held together by gravity.

The largest galaxies have 1 trillion stars or more. The smallest have only about 10 million stars. Our own galaxy, sometimes called the ``Milky Way Galaxy'', contains about 200 billion stars. This is approximately half the number of Oreo cookies that have ever been baked. Think of it - two Oreos for every star in our galaxy. (To keep the lawyers off my back, I point out that ``Oreo'' is a registered trademark of Nabisco, Inc.)
What do galaxies look like? Here's a brief picture show. Click on the small images to get a larger view.

This is the Andromeda Galaxy, also known as M31. It is the closest large neighbor to the Milky Way Galaxy, being about 700 kpc away. (1 kpc = 1 kiloparsec = 1000 parsecs = 3260 light years)

This is the Whirlpool Galaxy, also known as M51. Note the distinctive spiral structure which it displays. In the larger image, you can see a neighboring galaxy just above M51.

This is the galaxy M87. Unlike M31 and M51, it is an ellipsoidal blob rather than a flattened disk.

There are more than 100 billion galaxies visible to our telescopes. However, only four of them are visible to the naked eye:

The distance between neighboring stars in a galaxy is about 10 million times the diameter of a star. By contrast, the distance between neighboring galaxies is typically less than 100 times the diameter of a galaxy.

(2) After long debate, the size and shape of our own galaxy were determined.

Ancient Greek astronomers called the Milky Way, the faint band of light visible in the night sky, ``galaxias kuklos'' (in English, the ``milky circle''). This name is the origin of the term ``galaxy''. The ancient Greeks thought that the Milky Way was made of a luminous fluid embedded in the celestial sphere.

In the early 17th century, Galileo turned his telescope on the Milky Way, and discovered that it was not a continuous fluid, but consisted of innumerable faint stars, each too dim to be seen individually.

Galileo's discovery led to the hypothesis that the Sun is embedded within a thick disk of stars. The Milky Way occurs where we look in the direction parallel to the faces of the disk; in this direction we see more stars, and their light adds together to make the continuous band of light that we call the Milky Way.

William Herschel (discoverer of Uranus) and his sister Caroline (discoverer of many comets) made the first attempt to accurately determine the shape of our galaxy. They counted the number of stars along 683 lines of sight leading away from the Sun. If they saw few stars, they concluded that the edge of the galaxy was very near in that direction. If they saw many stars along a line of sight, they concluded that the edge of the galaxy was far away in that direction. Their conclusion: the galaxy is shaped like an irregular grindstone (or thick disk). A cross-section of the galaxy, as plotted by the Herschels is shown below (click on the image for a larger version). The Sun is slightly off-center to the left.

In the early 20th century, a Dutch astronomer named Jacobus Kapteyn refined the Herschels' technique. He concluded that the galaxy was a disk about 3 kiloparsecs thick and 17 kiloparsecs in diameter (about the shape of an Oreo cookie). He also believed that the Sun was near the center of the galaxy.

Both the Herschels and Kapteyn were WRONG!! The Sun is nowhere near the center of the galaxy, and the galaxy is significantly larger than Kapteyn thought. Kapteyn thought that because he couldn't see any stars farther than 8 kpc away, that there were no stars farther than 8 kpc away. In fact, the scattering of light by interstellar dust keeps us from seeing more than 8 kiloparsecs away in the disk of our galaxy.

The (approximately) correct size and shape of our galaxy were determined by Harlow Shapley in 1920. Shapley noted that the globular clusters that swarm around our galaxy all tend to lie on one side of the sky, centered on the constellation Sagittarius. If the Sun were at the center of the galaxy, we would see roughly the same number of globular clusters in all directions. Therefore, Shapley concluded that the Sun is NOT at the center of the galaxy; the center lies in the direction of Sagittarius, around which the globular clusters are distributed.

Shapley was also able to measure the distances to globular clusters. Globular clusters are too far away to measure their distances by parallax. However, globular clusters contain RR Lyrae variable stars. Every RR Lyrae star (as described in the lecture for Wednesday, January 29) has a luminosity of about L = 80 Lsun Measure the average apparent brightness b of an RR Lyrae star in a globular cluster, and you can find the distance d to that globular cluster from the relation
L = 4 pi d2 b .

Shapley's plot of the distribution of globular clusters is shown below. The yellow blotch where the axes cross is the position of the Sun. The red X off to the right is the center of the galaxy, as defined by the globular clusters.

Shapley concluded that the center of the galaxy is 17 kiloparsecs away. However, Shapley neglected to take into account the dimming of globular clusters by intervening dust.

The actual distance from the Sun to the center of the galaxy is 8 kiloparsecs = 8000 parsecs = 26,000 light years.

Copernicus deflated human egos by pointing out that the Earth is not the center of the Solar System. Shapley deflated our egos still more by pointing out that the Solar System is not the center of the galaxy. We are out in the suburbs, 3/4 of the way from the center to the edge of the galaxy's disk.


(3) After long debate, it was shown that our galaxy is one of billions of other galaxies.

In old textbooks, the Andromeda Galaxy is referred to as the Andromeda Nebula. Through a small telescope, the Andromeda Galaxy looks like yet another faint, fuzzy patch, like the Orion Nebula, the Ring Nebula, or the Crab Nebula. It took a long time to figure out which nebulae (like the Orion, Ring, and Crab Nebulae) consisted of glowing gas within our own galaxy, and which nebulae (like Andromeda) were galaxies in their own right.

Lord Rosse (mid 19th century), using what was then the world's largest telescope, determined that some nebulae were spiral nebulae; that is, as revealed by Rosse's sketch (below left), they have a swirling spiral structure.

[In the above illustration, the left-hand image is Rosse's sketch of the Whirlpool Galaxy (M51) - the right-hand image is a bad reproduction of a modern photograph of the same galaxy.]

For many decades, there were two viewpoints on the nature of spiral nebulae.

The debate was resolved by Edwin Hubble in 1924. Using what was then the world's largest telescope (the 100-inch on Mount Wilson, near Los Angeles), he discovered Cepheid variable stars in the Andromeda Nebula. The Cepheid stars were apparently very faint, which meant that the Andromeda Nebula was outside our own galaxy, and constituted a galaxy of its own.

From the Cepheid period-luminosity relation, the distance to the Andromeda Galaxy is found to be 700 kpc = 2.3 million light years.

The Andromeda Galaxy is the most distant object visible to the naked eye.
Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Feb 14

Copyright 2003, Barbara Ryden