Universe Today

Most galaxies can be categorized by their shape. Our own Milky Way is a spiral galaxy, for example, and the largest galaxies in the Universe are elliptical galaxies. But some galaxies defy categorization. These are the irregular galaxies, and each one is unique in shape, age and structure.

Irregular galaxies are often chaotic in shape, with no central bulge or spiral arms. Although they used to have a more familiar shape, a dramatic collision with another galaxy has distorted their shape.

Astronomers maintain two classifications of irregular galaxies. Irr-I galaxies have some structure, but they’re still distorted enough that they can’t be classified as spiral, elliptical or lenticular shaped. Irr-II galaxies don’t have any structure at all.

The nearby Magellanic Clouds were once thought to be irregular galaxies. Although astronomers have detected a faint barred spiral shape.

There’s only one irregular galaxy in the Messier catalog of objects, and that’s M82; also known as the Cigar Galaxy. It’s located in the constellation Ursa Major about 12 million light-years away, and is famous for its heavy amounts of star formation. In fact, in infrared light, M82 is the brightest galaxy in the sky. Even in visible light, it’s 5 times brighter than the Milky Way.

We have written many articles about galaxies for Universe Today. Here’s an article about Hubble’s recent image of irregular galaxy M82.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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Our own Milky Way is classified as a spiral galaxy. But that’s just one of many classification of galaxies. One of the most common types are elliptical galaxies, named because they have an ellipsoidal (or egg) shape, and a smooth, almost featureless appearance.

Elliptical galaxies are usually large, containing hundreds of millions to trillions of stars. The biggest galaxies in the Universe are elliptical galaxies. They’re the result of many collisions between smaller galaxies, and all these collisions have destroyed the delicate spiral structure that we see in our own galaxy.

And they’re usually old. Elliptical galaxies look redder than spiral galaxies like the Milky Way. That’s because they contain old, red stars and have very low rates of star formation. All of the available gas and dust was already used up in the past, and now all that remains are these old red stars. They also have large populations of globular star clusters.

Elliptical galaxies are usually found in the most violent places in the Universe, like at the heart of galaxy clusters and in compact groups of galaxies. In these places, elliptical galaxies have had an accelerated life, with many galaxy mergers and several periods of star formation. These constant mergers and collisions increased their size and used up all the gas available for star formation.

The smallest dwarf elliptical galaxies are no larger than a globular cluster and can contain a mere 10 million stars. The largest elliptical galaxies can have well over 10 trillion stars. The largest known galaxy in the Universe, M87, is an elliptical galaxy.

We have written many articles about galaxies for Universe Today. Here’s an article about an ancient elliptical galaxy, and another elliptical galaxy where a black hole halted its formation.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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One of the most beautiful images ever taken by the Hubble Space Telescope is the Sombrero Galaxy; also known as M104 or NGC 4594. This is an unbarred spiral galaxy located in the constellation of Virgo about 29 million light-years away.

Some of the defining features of the Sombrero Galaxy are its bright nucleus, large central bulge, and a prominent dust lane in its disk. The galaxy is seen nearly edge on, and so it has the appearance of a Sombrero hat. Since the galaxy has an apparent magnitude of +9.0, it’s easily visible in amateur telescopes; but too dim to see with the unaided eye.

The dark dust lane that you can see in the Hubble image is a symmetric ring that encloses the bulge of the galaxy. Astronomers have detected that it mostly contains hydrogen gas and dust. The bulk of star formation that occurs in the Sombrero Galaxy is happening within this ring.

As with our own Milky Way, astronomers have detected a supermassive black hole at the heart of the Sombrero Galaxy. Based on the speed of the stars orbiting around it, astronomers have calculated that it must have a mass of at least 1 billion suns. This is one of the most massive black holes detected in nearby galaxies.

If you want to look for the Sombrero Galaxy yourself, you’ll need good pair of 7×35 binoculars or a 4-inch telescope. The galaxy is located 11.5 degrees West of the star Spica, and 5.5 degrees northeast of Eta Corvi. With a medium sized telescope you can distinguish the bulge from the disk, and with a large telescope you should be able to see the dark dust lane.

We have written many articles about galaxies for Universe Today. Here’s an article about how the Sombrero Galaxy was imaged in three different wavelengths, and by three different observatories.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

To see most galaxies, you need at least a small telescope. But you can see the enormous Andromeda Galaxy, or Messier 31, with your own eyes; if you know where to look. The Andromeda Galaxy is located in the Andromeda constellation, and named after a princess in Greek mythology.

Andromeda is the largest galaxy in the Local Group, which includes the Milky Way, the Triangulum Galaxy, and dozens of smaller dwarf and irregular galaxies. A recent estimate gave Andromeda 700 billion solar masses. Our Milky Way is only 80% the mass of Andromeda.

The Andromeda galaxy was first observed by Persian astronomers, thousands of years ago, and was later cataloged by Charles Messier in 1764. He classified it as M31. In 1912, astronomers calculated its speed to be 300 kilometers per second, moving towards the Sun. Edwin Hubble first calculated the distance to Andromeda, by detecting cepheid variables in the galaxy. He measured that it was 450 kpc, or 2.5 million light-years away; well outside the Milky Way galaxy.

Recent estimates have calculated that Andromeda Galaxy is about 220,000 light-years in diameter, almost twice the estimate diameter of the Milky Way.

While other galaxies are moving away from us, Andromeda is on a collision course with the Milky Way. Our two galaxies will collide with one another in about 2.5 billion years, and begin forming a giant elliptical galaxy. It’s known to have 14 dwarf galaxies orbiting it in various stages of merger.

We have written many articles about Andromeda Galaxy for Universe Today. Here’s an article about the constellation Andromeda, which contains the Andromeda Galaxy, and here’s a beautiful picture of Andromeda captured by Spitzer.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

The Milky Way is just one galaxy located in a vast cluster of galaxies known as the Local Group. This group contains more than 50 galaxies (mostly dwarf galaxies). The total size of the Local Group is 10 million light-years across, and it’s estimated to have a mass of 1.29 billion solar masses. The Local Group is just one collection of galaxies in the even bigger Virgo Supercluster.

The largest, most massive galaxies in the Local Group are the Milky Way, Andromeda and the Triangulum Galaxy.

Each of these galaxies has a collection of satellite galaxies surrounding them. For example, the Milky Way has Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major Dwarf.

Andromeda has satellite galaxies M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph, Cassiopeia Dwarf, And VIII, And IX, and And X.

The Traingulum galaxy might be a satellite to Andromeda, and it might also have the Pisces Dwarf as a satellite.

The other members of the Local Group, not associated with another galaxy, include: IC10, IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.

The first astronomer to identify the Local Group was Edwin Hubble, who called the collection the Local Group in his book, The Realm of Nebulae. Of course, at this time Hubble didn’t know that they were distant galaxies, separate from our own Milky Way, so he called them nebulae.

We have written many articles about galaxies for Universe Today. Here’s an article about a dwarf galaxy falling into the local group, and here’s an article about how the Universe doesn’t seem to be expanding evenly.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

The space between stars is known as interstellar space, and so the space between galaxies is called intergalactic space. These are the vast empty spaces that sit between galaxies. For example, if you wanted to travel from the Milky Way to the Andromeda galaxy, you would need to cross 2.5 million light-years of intergalactic space.

Intergalactic space is as close as you can get to an absolute vacuum. There’s very little dust and debris, and scientists have calculated that there’s probably only one hydrogen atom per cubic meter. The density of material is higher near galaxies, and lower in the midpoint between galaxies.

Galaxies are connected by a rarefied plasma that is thought to posses a cosmic filamentary structure, which is slightly denser than the average density of the Universe. This material is known as the intergalactic medium, and it’s mostly made up of ionized hydrogen. Astronomers think that the intergalactic medium is about 10 to 100 times denser than the average density of the Universe.

This intergalactic medium can actually be seen by our telescopes here on Earth because it’s heated up to tens of thousands, or even millions of degrees. This is hot enough for electrons to escape from hydrogen nuclei during collisions. We can detect the energy released from these collisions in the X-ray spectrum. NASA’s Chandra X-Ray Observatory – a space telescope designed to search for X-rays – has detected vast clouds of hot intergalactic medium in regions where galaxies are colliding together in clusters.

We have written many articles about galaxies for Universe Today. Here’s an article about how intergalactic dust might be messing up observations, and here’s an article about a cosmic hurricane in a starburst galaxy.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

When astronomers first started using radio telescopes in the 1950s to study the Universe, they discovered a strange phenomenon. They found objects that shone brightly in the radio spectrum, but they couldn’t see any visible object associated with them. They called them quasi-stellar radio sources, or “quasars” for short.

Within a decade of their discovery, astronomers learned that these quasars were moving away at tremendous velocities. This velocity, or red-shift of their light, indicated that they were billions of light-years away; beyond the capabilities of most optical telescopes. It wasn’t until the 1960s when a quasar was finally tied to an optical object, a distant galaxy.

Since then, thousands of quasars have been discovered, but astronomers had no idea what they were. Finally in the 1980s, astronomers developed unified models that identified quasars as active galaxies. The bright radiation coming from them is because of the accretion disks surrounding the supermassive black holes at their centers. We see a quasar when a supermassive black hole is actively feeding on the surrounding material.

Since our own Milky Way has a supermassive black hole, it’s likely that we have gone through many active stages, whenever material is falling into the black hole; our galaxy would be seen as a quasar. But other times, like now, the supermassive black hole is quiet.

With new powerful telescopes, astronomers have observed that some quasars have long jets of material firing out from the center of the galaxy. These are channeled by the magnetic fields created by the supermassive black hole’s rotation in the accretion disk. The most luminous quasars can exceed the radiation output of an average quasar.

We have written many articles about quasars for Universe Today. Here’s an article about the first triple quasar ever found, and some hidden quasars… found!

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.