Category: Guide to Space

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The Milky Way is a spiral galaxy, measuring 100,000 light-years across, but only 1,000 light-years thick. Imagine a spinning record, but made of stars. The galactic equator is the halfway point between the top and bottom of that disk. Imagine you were traveling in a spaceship through the galactic plane. The moment you cross the midpoint of the disk, that’s the galactic equator.

Astronomers measure the position of an object in the sky using a coordinate system that’s sort of like latitude and longitude on Earth. They use two numbers: right ascention and declination. Right ascention gives a position east/west in the sky, and declination measures north/south. But there’s another measurement system that some astronomers use called the galactic coordinate system, which uses the galactic equator of the Milky Way.

This system imagines the Sun at the center, with 0-degrees being a line drawn straight from the Sun to the center of the Milky Way. You can then measure locations in a circle around the galactic equator. You can also measure locations above and below the galactic equator.

The north galactic pole is perpendicular to the galactic equator – 90-degrees above the equator, and the south galactic pole is below.

You might be interesting to know that the Solar System bobs up and down above and below the galactic equator. It takes 64 million years to complete a full cycle going above and below the galactic equator. If you’re heard that the Solar System is supposed to cross the galactic equator in 2012, don’t worry, that’s a myth. It takes 64 million years to complete that cycle, so there’s no way to know exactly when it will actually cross the galactic equator.

Although there’s no danger of crossing the galactic equator, some researchers think that there might be a danger of rising above the galactic disk.

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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The Milky Way is a vast spiral galaxy, shaped a bit like a spinning record; just one that measures 100,000 light-years across and only 1,000 light-years thick. Imagine you were below the Milky Way, and passed through the disk of stars above it. That moment when you’re halfway through the 1,000 light-year thickness of stars? That’s the galactic plane.

Astronomers actually use a coordinate system to measure positions in the Milky Way, starting with the Sun as the center point. No, we’re not actually at the center of the Milky Way, we’re actually off to the side, but this makes the measurement easier. They draw a line from the Sun to the center of the Milky Way, and that defines the 0-degree point, and then coordinates are measured within the galactic plane. You can have galactic latitude and longitude.

Have you heard anyone mention that the Sun is supposed to be crossing the galactic plane in 2012? Yeah, that’s a myth. Here’s the thing. The Sun does bob up and down in the galactic plane. Sometimes we’re above the plane, and then other times we’re below the plane. But that cycle takes 64 million years to complete! It’s impossible to define the exact moment of when the Solar System will pass exactly through the galactic plane.

And astronomers don’t think that anything special will happen when the Solar System does pass through the galactic plane. In fact, it’s the times when the Earth is above or below the galactic plane when we might be at risk. A recent scientific study correlated those times with large extinction events in the Earth’s history. It’s possible that the Milky Way’s magnetic field protects the Earth from intergalactic radiation and cosmic rays, and when we’re significantly above or below the galactic plane, life on Earth suffers more damage from space radiation.

But that’s just a theory.

So, to summarize, the Solar System won’t be passing through the galactic plane in 2012. There’s no easy way to know exactly when that’ll happen, and there’s absolutely no way to give that a specific date. And even when we do pass through the galactic plane, there’s no risk to our planet.

Here’s a link to an article on Universe Today about possible extinction events when the Earth is above or below the galactic plane.

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.

Sources: EarthSky.org, NASA

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The distance to the Andromeda Galaxy is 2.54 million light-years, or 778 kiloparsecs.

The Andromeda Galaxy can be seen with the unaided eye, so skywatchers have been observing it for thousands of years. Charles Messier cataloged it as M31 in his 1764 list. Back then, astronomers thought that Andromeda was a nebula, and based on its size, Messier guessed that it was only about 2,000 times further than the star Sirius.

Astronomers discovered variable star called novae in Andromeda in 1917, and quickly realized that they were 10 times less bright than similar objects in the Milky Way. Astronomers Heber Curtis proposed that Andromeda was a separate “island universe”, located about 500,000 light-years away. Edwin Hubble ended the controversy once and for all in 1925 when he identified Cepheid variable stars in Andromeda, and calculated that the galaxy was actually 1.5 million light-years away.

Modern astronomers are continuing to calculate the distance to Andromeda. In 2003, astronomers calculated that Andromeda is 2.57 million light-years away. And in 2004, astronomers redid Hubble’s Cepheid variable calculations, and determined that Andromeda was 2.51 million light-years. Another group used a different technique in 2005 to calculate that Andromeda was 2.52 million light-years away. And yet another technique in 2005 put it at 2.56 million light-years away. And so, the agreed distance of 2.54 million light-years is an average of the distances measured so far.

There are several galaxies closer to Earth than Andromeda. The Large Magellanic Cloud is only 160,000 light years away, and the Canis Major Dwarf Galaxy is a mere 25,000 light-years from Earth. But Andromeda is the largest grand spiral galaxy to us.

We have written many articles about galaxies for Universe Today. Here’s another article about the closest galaxies to the Milky Way.

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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Look across the Universe, and you’ll see that almost everything is rotating. The Earth rotates on its axis as it orbits the Sun. And the Sun itself is rotating. As you can probably guess, we even have galaxy rotation with our Milky Way galaxy.

Our galaxy is rotating incredibly slowly, however. It takes the Sun 220 million years to complete a single orbit around the galaxy. In the 4.6 billion years that the Sun and planets have been here, they’ve only rotated around the center of the galaxy about 20 times.

We know that galaxy rotation is happening because the Milky Way is a flattened disk, in the same way that the Solar System is a flattened disk. The centrifugal force from the rotation flattens out the galactic disk. All stars in the galactic disk follow roughly circular orbits around the center of the galaxy. Stars in the halo can have much different orbits and speeds.

The calculation of the high rotational speed of the galaxy led to the discovery of dark matter. If our galaxy contained just the matter we can see – planets, gas, etc – the galaxy rotation should cause it to spin apart. Instead, there’s much more mass holding the galaxy together. In fact, astronomers have calculated that the total mass of the galaxy is probably 10 times greater than the sum of all the stars in it. 90% of this is invisible dark matter, holding the galaxy rotation together. And only 10% is the regular matter that we can see. Our galaxy really has a mass of more than 1 trillion suns, and extends out more than 600,000 light-years; a third of the distance to the nearby Andromeda galaxy.

All the galaxies we can see are rotating. It’s this rotational force that counteracts the inward pull of gravity from all the galaxies. If galaxies didn’t rotate, they’d collapse inward and just join the supermassive black holes at the hearts of galaxies.

We have written many articles about galaxies for Universe Today. Here’s another article about the rotation of the Milky Way.

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.

References:
SEDS
http://www.astronomy.ohio-state.edu/~ryden/ast162_7/notes30.html

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Galaxies can range in size from having just a few million stars to well over a trillion stars. But have you ever wondered, what’s the largest galaxy in the Universe. The Universe is a big place, and we’ll probably never be able to see every single galaxy. So we can never know for sure what the biggest galaxy is. Instead, we’ll have to go with, what’s the largest galaxy that we know of?

The largest galaxies in the Universe are the giant elliptical galaxies. These are large, egg-shaped galaxies with trillions of stars. They’re formed through multiple collisions between smaller spiral galaxies of similar size. For example, when our own Milky Way collides with the same sized Andromeda Galaxy in a few billion years, the outcome will probably be a giant elliptical galaxy, with about a trillion stars.

The galaxies that can get the largest are the ones at the very center of galaxy clusters. Astronomers call these cD galaxies (for giant diffuse galaxies), or bright cluster galaxies. The grow by gobbling up any galaxy that comes too close to them, and since they’re at the center of a galaxy cluster, many galaxies get too close. In fact, these galaxies have a large space around them where astronomers can’t find any smaller galaxies; they’ve all been consumed by the larger galaxy.

A large cD galaxy can be 10 times brighter than the Milky Way, with about 100 times as much mass. They can have a diameter of 6 million light-years across (the Milky Way is about 100,000 light-years across).

An example of this is the central galaxy in the cluster Abell 2029.

It’s probably that there are even larger galaxies out there. And if they are there, you’ll find them at the center of the largest galaxy clusters.

We have written many articles about galaxies for Universe Today. Here’s an article about research into the galaxy cluster Abell 2029.

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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There are spiral galaxies and elliptical galaxies, but one of the strangest you’re ever going to see is a ring galaxy. One of the most famous examples of these is Hoag’s Object, discovered in 1950 by Art Hoag – but there are other examples as well.

And a ring galaxy really does look like a ring. There’s a bright central core, and then a large gap without much luminous matter, and then a bright ring containing hot, blue stars.

Astronomers think that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. The space between stars in a galaxy is vast, so when galaxies collide, the stars don’t actually crash into each other. Instead, it’s their gravity that makes a mess. In this situation, it’s thought that the smaller galaxy slices right through the disk of the larger galaxy. The gravity of the smaller galaxy collapses vast clouds of gas and dust, and creates a burst of star formation around the edge of the larger galaxy.

The change in gravity drastically affects the orbit of the stars in the larger galaxy. They orbit outward and bunch up into the bright starforming ring. This blue ring is continuing to expand outward, and astronomers believe that it only lasts for a few hundred million years before it begins disintegrating. Eventually only the bright galaxy core will remain.

In 2004, astronomers released an image of the ring galaxy AM 0644-741 to celebrate 14 years of service by the Hubble Space Telescope.

We have written many articles about galaxies for Universe Today. Here’s an article about a ring galaxy imaged by Hubble.

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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The “Comet Galaxy” is actually just one galaxy located in a distant galaxy cluster known as Abell 2667, located about 3.2 billion light-years away. A recent photograph captured by the Hubble Space Telescope showed this galaxy being torn apart into a comet shape by the intense gravity of galaxy cluster – and that’s how it got the nickname as the Comet Galaxy.

The observation of the Comet Galaxy and the rest of the galaxies in Abell 2667 helped astronomers understand why many galaxies are “gas poor”. Our own Milky Way has tremendous stores of gas and dust which are used for star formation. But other galaxies out there have very little gas which can be used for star formation.

The image of the Comet Galaxy by Hubble helped show that huge gravitational interactions between galaxies in massive clusters cause tremendous damage to the structure of a galaxy, and the amount of gas they have. Galaxies near the center of clusters experience the most damage of all, which galaxies at the outskirts are relatively unharmed. The galaxy collisions can distort the shape of galaxies, and even fling out “homeless stars” into intergalactic space.

Even though the Comet Galaxy’s mass is slightly greater than the Milky Way, it will lose all its gas and dust, and so not be able to generate stars later in life. It will become a gas-poor galaxy with an old population of red stars.

Because the Comet Galaxy is 3.2 billion light-years away, it can only really be seen with the Hubble Space Telescope. Even a powerful backyard telescope wouldn’t be able to locate it.

We have written many articles about galaxies for Universe Today. Here’s a news release from the Hubble Space Telescope about the Comet 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.

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It’s time to expand your mind and consider the largest structures in the Universe: vast collections of galaxies known as superclusters. There’s really nothing bigger in the Universe. The supercluster we live in is known as the Virgo Supercluster. It’s an enormous collection of more than a million galaxies, stretching across a region of space 110 million light-years across.

Our Sun is just one member of the Milky Way, and the Milky Way is part of a collection of galaxies known as the Local Group. This contains three large spiral galaxies: the Milky Way, Andromeda, and the Triangulum Galaxy, as well as a few dozen dwarf galaxies. The Local Group is just one member of the Virgo Cluster. This is a collection of 1200-2000 galaxies that stretch across 15 million light-years of space. And then, the Virgo Cluster is just one cluster in the Virgo Supercluster.

Although astronomers recognized that we were in a supercluster of galaxies in the 1950s, it wasn’t until the 1970s that astronomers actually mapped out the Virgo Supercluster’s shape; it has a flattened disk, somewhat like our galaxy itself. Our Virgo Cluster is actually an outlying group of the Virgo Supercluster.

The Virgo Supercluster is just one of millions of superclusters across the Universe. From the largest scales, they’re arranged in long filaments and walls surrounding even large voids of space where there are almost no galaxies at all.

We have written many articles about galaxies for Universe Today. Here’s an article about how the Universe isn’t expanding uniformly.

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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The key force in astronomy is gravitational attraction. Planets orbit stars, and stars are part of galaxies. But even galaxies come together into groups with dozens of members. One of the larger structures in the Universe are galaxy clusters; collections of thousands of galaxies. And our Milky Way is no exception. We’re a part of a much larger structure known as the Virgo Cluster.

The Virgo Cluster contains about 1300-2000 member galaxies, which are all connected together by mutual gravity. Astronomers estimate that it contains a total mass of about 1.2 quadrillion times the mass of the Sun. It covers a total volume of space with a diameter of 15 million light-years across.

Just like it’s name, the galaxies in the Virgo Cluster are mostly located in the constellation of Virgo, and one of the largest members of the cluster is the giant elliptical galaxy Messier 87. This monster is thought to have about 2.7 trillion solar masses all on its own.

The Virgo Cluster is just one member of an even larger structure: the Virgo Supercluster. This supercluster, also located in the constellation of Virgo is thought to have more than a million member galaxies, and stretches across a region of space 110 million light-years in size.

Superclusters are the largest structures in the Universe, and there are thought to be millions of them across the entire Universe. From the largest scales, these superclusters are stretched out into long filaments with large voids in between them. Seem from the longest distances, they would look like foam bubbles.

We have written many articles about galaxies for Universe Today. Here’s an article about the Virgo Cluster sucking in a distant galaxy, and here’s another article detailing a study of the Virgo Cluster.

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 an example of a grand spiral; a vast collection of 200 to 400 billion stars. Much smaller galaxies than our own are known as dwarf galaxies. They only contain a few billion stars, and have a fraction of the mass of the Milky Way.

A good example of a dwarf galaxy is the Large Magellanic Cloud, located about 160,000 light-years from Earth. It contains about 1/10th the mass of the Milky Way, and has about 10% of its stars. Two other dwarf galaxies are even closer to the Milky Way, and have been captured by our galaxy’s gravity. Other dwarf galaxies are just remnants that have been torn apart by the Milky Way’s gravity, and are currently being incorporated into the structure of our galaxy.

Some astronomers think that the largest globular cluster in the Milky Way, Omega Centauri, might have once been a dwarf galaxy that had its outer stars stripped away.

Just like their larger cousins, dwarf galaxies can be classified into three varieties: dwarf elliptical galaxies, dwarf irregular galaxies, and dwarf spiral galaxies.

The smallest dwarf galaxies in the Universe are known as ultra compact dwarf galaxies. These are a recently discovered class of galaxies not much more massive than a globular star cluster. They can be as small as 200 light-years across and contain about a hundred million stars. It’s thought that ultra compact dwarf galaxies are just the cores of dwarf elliptical galaxies that were stripped of gas and outlying stars.

Our Local Group of galaxies contains just three large spiral galaxies: Andromeda, the Milky Way, and the Triangulum Galaxy. All of the others are dwarf galaxies of varying sizes.

We have written many articles about galaxies for Universe Today. Here’s an article about a companion dwarf galaxy that’s almost invisible.

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.