Monday, July 14, 2008

Globular Clusters

Drawing of Hercules Cluster M-13

Drawing of Hercules Cluster M-13 from Recreations in Astronomy by H.W. Warren, D.D., 1896

Globular clusters are briefly mentioned in Star Clusters. They are groups of stars that are held together by their mutual gravity. They tend toward a globe shape overall, which is why they're called "globular." The number of stars in a globular cluster ranges from a few thousand to a few million in the largest ones.

Globular clusters are like mini-galaxies. They orbit their host galaxy, but to some degree are separate from it. The globular clusters in the Milky Way orbit the core of the galaxy, or around the rim. This means that they are well distributed through the sky, but lots of them are visible when looking toward the center of the galaxy (in Sagittarius, the Archer.)

Bright, Easy Objects

Globular Clusters are bright and easily seen even on nights when other objects like galaxies and nebulas are washed out by moonlight, light overcast, or city lights. You can see them in binoculars, it doesn't take a telescope to enjoy them. I had a hobby for a couple of years of seeing how many globulars I could see with 7x35 binoculars. I collected over 40 before I got a new telescope and stopped looking regularly.

Any telescope with a good mount can do a good job of showing off the bright globular clusters. If you have a telescope of 12" (250mm) or greater aperture, you can observe globular clusters in other galaxies as well as those in the Milky Way. The Andromeda galaxy (M31) and the galaxy in the Triangle (M-33) make good targets for extra-galactic globular hunting.

What to Look For

When you observe a globular, it's interesting to look for a few things. First, see how far out the globular's stars appear to go. Then, notice how quickly the cluster seems to go from outlying individual stars to the tight, dense knot of stars at the center. This is called the "condensation" of the cluster. Does it condense rapidly, or slowly, or does it have scattered stars up to a certain point then suddenly appear to become a solid globe of stars?

Next look at the overall appearance of the globular. Does it have wispy tendrils of stars? Does it look like a patch of mist? Does it look like a glowing ball of jewels? Or what?

Makes notes of your observations. It's interesting to compare and contrast different globulars, as well as to see how different they appear in different observing instruments.

Famous Globulars

Here are a few of the globulars most popular with amateur observers:

M-13, the Hercules Cluster, is visible with just your eyes under dark skies. It's in the Keystone which makes it easy to find.

M-2 in Aquarius is a beautiful object in binoculars or telescopes.

M-4 in Scorpius is also visible in a dark sky, it's near Antares, which makes it easy to find.

Omega Centauri

Omega Centauri is large and bright. It lies far to the south for mid-northern latitudes, but is a great object for binoculars and telescopes when it is visible.


Globular Cluster M-80

Globular Cluster M-80, one of the densest clusters.



Saturday, July 12, 2008

Star Clusters

There are two types of star clusters, "open" clusters and "globular clusters." Globular clusters are like mini-galaxies. The gravity of the stars in them hold them together and pull them into a globe shape (which is why they got the name "globular.")

Open clusters are clusters of stars that were formed together and are still near each other. The mutual gravity of the stars usually isn't enough to hold them together, so the stars in open clusters gradually drift away from each other as they follow their own path through the galaxy. For the several million years that they are near each other, they can make a beautiful sight in the sky here on Earth.

The stars start out near each other as a result of having formed out of the same cloud of dust and gas in the galaxy. These clouds are many light-years across, and anywhere from a few to a few hundred stars can form out of a single cloud (the numbers are usually from a few to a few dozen stars.) Once stars form they blow the leftover gas and dust away from themselves. Sometimes this causes new clumps in the cloud that form into more new stars. At some point the available gas and dust becomes too little for form new stars, and it gets thin enough that the stars can be seen from outside the cloud. The leftover gas and dust may surround the young stars for quite some time, adding to the beauty of the cluster.

Star formation in dust clouds in a remote galaxy.
Here is a recent NASA image of massive star formation going on in a remote galaxy. Over 4000 new stars per year--ten times what we get in the Milky Way!

The Pleiades, M45, Star Cluster in Taurus
The Seven Sisters, or The Pleiades, a star cluster that's easy to see with the naked eye in Taurus. You can see some of the left over gas and dust as nebulosity (cloudiness) in this image.

Friday, July 11, 2008

How Far?

When I show things off at a star party, we usually look at things that cover a long range of distances, from near to far.

Very Close: 1-2 Light Seconds Away

During the day, we'll sometimes put a telescope on a nearby landscape feature such as a mountain top or a cell tower. The distance to these objects is familiar to most people. It's usually fractions of a mile to a few hundred miles. Astronomically speaking, these objects aren't at all distant.

Close: 1-6 Light Hours Away

We also show off the Moon both in day and night. It is only a quarter million miles away. It takes light about one and a quarter seconds to go this distance, so we might say that it is just over a light-second away.

The Sun and bright planets range in the millions of miles. The sun is about 93 million miles away, planets range from a bit under half this distance to several times this distance. The distance to the sun is about eight light minutes. The planets range from about three light minutes (a very close pass with Venus) to just about six light hours (Pluto and Eris.)

Nearby: 4-25 Light Years Away

The next step in distance takes us from measurements in light hours to measurements in light years. The nearest star is a component of the triple-sun Alpha Centauri system, called Proxima Centauri. It's just over four light years away. This is about 37,000 light hours away. The nearest stars are a whole lot further away than the solar system's planets, the visible comets, the asteroids, the Moon and Sun. Sometimes in art you'll see a star shown in front of the Moon (or within the horns of a crescent Moon.) This can never happen, they are millions of times further away than the Moon. (Yes, except for our own star, the Sun. But it's still hundreds of times farther away than the Moon.)

Nearby stars include Altair, in the Eagle, Sirius in the Big Dog, and Procyon in the Little Dog. Near range out to about 25 light years away or so.

Middle Distance: 25-100,000 Light Years Away

Stars in our galaxy range from these nearby neighbors to the stars in the globular clusters. These are about as far away as you can get while still being in the Milky Way galaxy. The ones you can see can be about as far away as 80,000 light years. Most of the stars you'll see at night are far closer, however, from the nears stars above out to distances of a few hundred light years.

This range includes the nebulas, like the Ring Nebula, Orion Nebula, and Lagoon Nebula. It also includes the star clusters.

Far: Millions of Light Years Away

Here we get to the galaxies. The nearest galaxy to ours (not counting small companions to the Milky Way, like the Magellenic Clouds) is the Andromeda Galaxy, at about 3 million light years distance. A whole bunch of galaxies in the Virgo Cluster is about 50 million light years away. These objects are all far further from us than any of the individual stars, clusters, or nebula you will see through a telescope that are part of our galaxy. Telescopes with mirrors or main lenses larger than 10 inches or so can show individual items in the closest galaxies, like the globular clusters and nebulas in the Andromeda galaxy. But normally any of these you see are in our galaxy, and normally over 100 times closer than even the nearest galaxy.

If you want an idea of what you can see in different sizes of telescopes, see What Can I See in My Telescope?

Astro Basics

Welcome to Astro Basics! I'm an amateur astronomer with over 40 years of experience. I have been sharing the sky through teaching and telescopes for all that time as well. I regularly take out my telescope to public places to show folks the amazing universe we live in.

When I do this, I get all sorts of questions about the cosmos. Many of them are common questions, some of them are uncommon, but likely to be interesting to anyone who thinks of them or hears them.

I'll be trying to bring some of those things to you here.

If you're interested in information about buying your own telescope, or how to use one you've got, I recommend visiting my other site:First Telescope Guide.