Spring is galaxy season. If you have a small telescope, this is a good time of year to get it out to find galaxies in the sky. Because most will appear as dim fuzzy blobs, a detailed star chart is essential for locating them. However, with a little practice you will be able to notice the variety of shapes that galaxies come in.
The distribution of galaxies in the sky tells us a great deal about the universe we live in. What is known as the cosmological principle says the universe, at least on a really big scale, is both homogeneous and isotropic. Being homogeneous just means that the distribution of galaxies near the Milky Way is similar to everywhere else we might be able to look. In other words, things are pretty much evenly distributed. That the universe is isotropic means that there are no preferred directions to look in – no direction looks like it is toward a middle or an edge. And that isotropic principle would hold true from any other place we could look.
But we wouldn’t have the cosmological principle if we didn’t have a way to know how galaxies are actually positioned relative to each other. We need to know not only their direction, but also their distance. We can deduce some qualitative distance information using the idea that galaxies that appear larger and brighter are most likely closer to us. This turns out to be true, but it would be nice to have some way of measuring those distances more precisely. That’s where variable stars come in.
Last month, I talked about the eclipsing binary star Algol as a type of variable star. Another class of variable stars, called Cepheid variables, pulsate very regularly in brightness with a period of from several days up to several weeks.
Cepheid variables are a good example of what is called a standard candle. Imagine seeing a candle in the distance. If you know how bright the candle is when it is nearby, you can figure out how far away it is simply by observing how bright it appears in the distance. By studying numerous, relatively nearby Cepheid variables, it was determined that a close relationship exists between the period of pulsation and their overall luminosity. This close relationship is what makes Cepheids such good standard candles, and that means we can tell how far away these stars are by measuring their period of pulsation.
Fortunately, Cepheids are among the most luminous stars there are, so we can see them even in other galaxies. The discovery of Cepheid variables in the relatively nearby Andromeda galaxy was the first direct evidence that what was then known as the Andromeda nebula was not a part of the Milky Way, but was in fact another galaxy.
This monthM31, or the Andromeda galaxy, can be seen in the northwestern sky in the early evening. It will be about 15 degrees to the left of the W-shape of Cassiopeia and sets by 11 p.m. From a dark location, it is visible to the naked eye.
If you want a good look at a Cepheid variable, look no farther than Polaris. The North Star changes brightness with about a four-day period, but you would need an instrument to detect those small changes. If you do get your telescope out to look at Polaris, it is also a double star with a companion that can easily be seen through an 8-inch telescope.
The vernal equinox this year is March 20. This first day of spring signals when the daylight hours will start to exceed the nighttime hours.
A day later, March 21, there is another full supermoon. This is the third one in a row, so excuse my yawn.
Tonight (Saturday), we all get to spring the clocks forward, and you get to read another of my complaints about this annual waste of prime-time dark-sky opportunities. Grumble, grumble, grumble.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.