Universe Today

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The Andromeda galaxy is the closest spiral galaxy to the Milky Way (though it’s not the closest galaxy). It’s the most distant object you can easily see with the naked eye (under good observing conditions). Andromeda is 220,000 light years in diameter, and is one of the 35 objects that make up what is called the Local Group. Andromeda lies, of course, in Andromeda constellation.

The Andromeda galaxy (also known as  Messier 31, M31, or NGC 224) could be considered the big brother of the Milky Way, as it contains over a trillion stars (compared to our 200-400 billion), and is approximately 220,000 light years across to our 100,000. Andromeda and the Milky Way formed at roughly the same time – 13.5 billion years ago – near the beginning of the Universe. Our galaxy is thought to look much like Andromeda. Both Andromeda and the Milky Way got to their current size by eating up other galaxies they collide with. The expansion of the Universe causes most galaxies to move away from us, but Andromeda and the Milky Way are actually headed towards each other.

Andromeda and the Milky Way are good neighbors, but eventually our neighbor is going to move in with us – the Milky Way and Andromeda are approaching each other at 200 kilometers per second, and will eventually collide. There’s no need to panic, though, as Andromeda is over 2 million light years away, and the collision won’t happen for another 2 or 3 billion years. Astronomer John Dubinski of the University of Toronto has an excellent animated simulation from multiple perspectives of what this galactic dance could look like.

The collision between Andromeda and the Milky Way won’t be catastrophic, and after about 5 billion years from now the resulting galaxy will have settled down into an elliptical galaxy. There is a small chance, though, that the Sun won’t be part of this new “Milkomeda” galaxy.

Fraser and Pamela discuss how the collision between the Milky Way and Andromeda will look from Earth in the September 28th, 2008 episode of Astronomy Cast, and the Milky Way in Episode 99.

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The small constellation of Pictor resides just south of the ecliptic plane and was created by Nicolas Louis de Lacaille. It was adopted by the International Astronomical Union and accepted as one of the permanent 88 modern constellations. Pictor covers approximately 247 square degrees of sky and ranks 59th in size. It has 3 main stars in its asterism and contains 15 Bayer Flamsteed designated stars within its confines. Pictor is bordered by the constellations of Caelum, Carina, Columba, Dorado, Puppis and Volans. It is visible to all observers located at latitudes between +26° and ?90° and is best seen at culmination during the month of January.

Because Pictor is considered a “new” constellation, it has no mythology associated with it – but Nicolas Louis de Lacaille was a man of science and arts. The constellation names he chose to add to his southern star catalog – Coelum Australe Stelliferum – favored this love of technological advances and all things in the field, therefore Pictor was once added as “Equuleus Pictoris”, the “artist’s easel”, but was later shortened to just Pictor when added permanently to the modern constellation charts.

Let’s begin our tour of Pictor with binoculars and its brightest star – Alpha Pictoris – the “a” symbol on our map. It is a class A subgiant star which resides almost 100 light years away from Earth. At close to a billion years old, it is around 3 times larger than our own Sun, yet it rotates over 100 times faster. Alpha is a star that shouldn’t produce X-rays – but does. What’s going on? Perhaps it has a small companion star that’s waiting to be discovered!

Keep your binoculars in hand and hop to Beta Pictoris – the “B” symbol. Located about 64 light years from our solar system, Beta is the key player in a moving star group. This is a stellar association of young stars which share the same motion through space and have the same age. But that’s not all that Beta has going for it. The Beta Pictoris system is very young – only 8-20 million years old – and already in the main sequence stage of stellar evolution. While that in itself isn’t peculiar, what’s curious is an excess of infrared emission compared to normal stars of its type. It would appear that Beta has large quantities of dust! According to detailed studies, a large disk of dust and gas has been found orbiting Beta and was the first to ever be imaged. Inside they found the presence of several planetesimal belts and cometary activity… and there are indications that planets may have formed within this disk and that the processes of planet formation may still be occurring! In November 2008, the European Southern Observatory (ESO) published a press release announcing that a planet matching previous predictions may have been imaged in orbit around Beta Pictoris in the plane of the debris disk. If the physical association of the detected object with Beta Pictoris is confirmed, it would be the closest planet to its star ever photographed. How far apart you ask? Tthe observed separation between the parent star and the planet is roughly the same as the distance between Saturn and the Sun. Too cool….

Now, take out your telescope and have a look at Theta Pictoris – the figure “8” symbol. That’s right… We’ve got a multiple star system here! Theta Pictoris is a three part system, with each of the components all around 7th magnitude and well spaced enough to be easy for optics!

For a nice optical double star in binoculars, have a look at Eta Pictoris – the “n” symbol on our map. Although not gravitationally bound, it’s still a pretty pair!

While there is almost no deep sky to be observed in Pictor, you can still scope out Kapteyn’s Star. It is a class M0 subdwarf star which was discovered by Jacobus Kapteyn in 1897. Located just about 13 light years from Earth, this one has a high radial velocity, orbits the Milky Way in retrograde, and is the nearest halo star to the Sun! When Kapetyn first discovered it, it had the highest proper motion of any star known, later bowing to the discovery of Barnard’s star..

Don’t forget to have a look at variable star, R Pictoris, too!

Sources:
Chandra Observatory
Wikipedia
Chart provided by Your Sky.

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The mass of the Milky Way depends on what you consider this question to mean. If you are only talking about the visible part of the Milky Way – all the stars, gas and dust that make up the disk – then the Milky Way’s mass is between 200-600 billion times that of the Sun. We can’t just put the Milky way on a bathroom scale to get this number, however. This number is reached by counting the number of stars in the galaxy and assuming their mass is roughly that of the Sun. The mass varies depending on where one defines the edge of the Milky Way to be.

But there is another way to check the heft of the Milky Way – by measuring how fast stars are rotating around the disk, the mass of the disk itself can be determined. In other words, the heavier the Milky Way is, the more of an effect gravity will have on the rotation, and the faster the stars will move through the disk. This number comes up to be a whopping 1-2 trillion times the mass of the Sun!  The most recent estimate from a study using information from the Sloan Digital Sky Survey measuring the velocity of over 2,4oo stars put the mass of the Milky Way and its halo at 1 trillion solar masses. Though astronomers don’t use kilograms when measuring such large objects as the Sun or galaxies, the Milky Way and its halo would be about 6 x 10^42 kilograms.

Where is all of this matter, if not in the stars? As with many contemporary mysteries in astronomy, the answer is dark matter. The Milky Way is thought to be home to a halo of dark matter – matter that cannot be detected except through its gravitational influence – which makes up approximately 80-90% of its mass. That’s right, the mass of the Milky Way that can be seen (through visible, X-ray, infrared, etc.) makes up only about 10-20% of its mass. This halo may extend out to as far as 300,000 light years from the galactic center.

For more information about the Milky Way, you can refer to Episode 99 of Astronomy Cast, visit the rest of our section here in the Guide to Space, or Swinburne Astronomy Online.

Source:
Sloan Digital Sky Survey

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The Milky Way is pretty hard to map, given that we live inside of it and have to peer through all of the dust and gas that lie inside the disk. Though we can’t get a picture of our galaxy from outside, we can create images and maps from computer modeling of the stars we see in the disk.

The Milky Way – until recently – was thought to be a barred spiral with four star-forming arms, named  Norma, Scutum-Centaurus, Sagittarius and Perseus. In June of 2008, images from NASA’s Spitzer Space Telescope revealed that the Milky Way is a barred spiral with only two major arms, demoting the Sagittarius and Norma arms to minor arms. The Sun lies in a minor arm, named the Orion Arm, or Orion Spur, sandwiched between the Sagittarius minor arm and Perseus arm.

Our galaxy is a large disk approximately 100,000 light years across. There’s a bulge in the center that is 12,000-16,000 light years thick, and is home to a black hole named Sagittarius A*. Other areas of the disk range between 2,300 and 2,600 light years in thickness.

The image above is a representation of what the Milky Way would look like from above. Of course, there are plenty of maps of the Milky Way as we see it from the Earth. Also, you can go outside on a clear night and see it splashed across the sky. Below is an image in the infrared, with the various regions marked (named because of the constellation in which they lie).

Plenty of maps are available all over the web in a variety of spectrum. Here’s a gallery of 9 images of the Milky Way in different spectra to get you started, and An Atlas of the Universe has maps of the Milky Way and Universe from different perspectives and distances. If you’re looking for interactive maps of the sky and Milky Way, Sky-Map.org, Google Sky and the downloadable Stellarium are all great resources to familiarize yourself with our Galaxy.

If you’re interested in learning more about other aspects of the Milky Way, Astronomy Cast has a whole episode devoted to it. You can also check out the rest of our resources in the Milky Way section of the Guide to Space.

Sources: NASA, Wikipedia