Author: Nicholos Wethington

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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

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The diameter of the luminous Milky Way is between 100,000 and 120,000 light years across, but that number could be much larger if you take into account dark matter. It all depends on where you measure the edge of the Milky Way to be…

If you are just measuring the normal matter we can see (in visible, infrared, X-ray and ultraviolet light), then the Milky Way is at least 100,000 light years across. The diameter is a bit larger (120,000 light years) if you take into account tidal streams – basically other galaxies the Milky Way is eating, such as the Sagittarius Dwarf Elliptical Galaxy.

But normal matter isn’t all that makes up the Milky Way. Simulations of our galaxy show that it has a “halo” of dark matter, which makes up about 10 times the mass of the visible matter in the Milky Way. Dark matter has never been directly observed, but is inferred due to its gravitational pull. This halo extends past the edge of the luminous part of the Milky Way, but the size of the halo has yet to be determined to a great degree of accuracy.

How do we measure the diameter of the Milky Way, given the fact that we live inside of it? We measure the distance to Cepheid variable stars. These are stars whose luminosity changes in a very predictable way because they puff up and shrink. Knowing the absolute luminosity of these stars allows us to measure their distance. Think of it this way: if you know how bright a flashlight is at 10 feet from you, and how that luminosity changes over distance, when it moves further away you can calculate that distance by determining how much dimmer the flashlight is.

Pamela talks about the diameter of the Milky Way, and how we measure it, in Episode 99 of Astronomy Cast. If you’re interested in learning more about variable stars – or even observing them – the American Association of Variable Star Observers is a great place to start.

If you were going to throw a birthday party for the Milky Way, how many candles would you put on the cake? What is the age of the Milky Way? Well, even though this is a difficult question to answer, either way you slice the cake you need a lot of candles. If you were to put a candle for each year the Milky way has aged, then you’d need between 10 and 13.6 billion candles. That would be mighty difficult to blow out all in one go.

The oldest stars in the Milky Way are 13.4 billion years, give or take 800 million years. This is somewhat close to what the age of the Universe is (which hovers around 13.7 billion years). By measuring the age of these stars, and then calculating the interval between their formation and the death of the previous generation of stars, we can come to an approximate age of the Milky Way as 13.6 billion years. Here’s a good article on how this process works.

The age of the Milky Way is determined by measuring the amount of beryllium present in some of the oldest known stars in the Milky Way. Hydrogen, helium and lithium were all present right after the Big Bang, while heavier elements are produced in the interiors of stars and dispersed via supernovae. Beryllium-9, however, is produced by collisions of cosmic rays with heavier elements.

Since beryllium is formed in this way, and not in supernovae, it can act as a “cosmic clock” of sorts. The longer the duration between the first stars that created heavier elements and the stars that make up globular clusters in the early Milky Way, the more beryllium there should be from the exposure to galactic cosmic rays. By measuring the beryllium content of the oldest stars in the Milky Way, the age of the Milky Way can be approximated.

This method is kind of like using radioactive decay of carbon-14 on Earth to determine the age of fossils. Radioactive decay of uranium-238 and thorium-232 gives an age of the Milky Way as similar to that of measuring the abundance of beryllium.

The age of the Milky Way is a tricky question to answer, though, because we can say that the oldest stars are 13.4 billion years old but the galaxy as we know it today still had to form out of globular clusters and dwarf elliptical galaxies in an elegant gravitational dance. If you want to define the age of the Milky Way as the formation of the galactic disk, our galaxy would be much younger. The galactic disk is not thought to have formed until about 10 – 12 billion years ago.

Here’s an article on how the bulge in the Milky Way may have formed earlier than the rest. Also, we’ve recorded a show all about the Milky Way on Astronomy Cast.

Source: ESO News Release

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The center of the Milky Way is a pretty happenin’ place. As with most other galaxies, there is a supermassive black hole there. Ours is named Sagittarius A* (pronounced “Sagittarius A-star”, abbreviated as Sgr A*). Not only does Sgr A* try to eat anything that goes near it, the area around it is a good place for new stars to form.

Since a black hole has such a huge gravitational footprint, it tries to suck up anything that comes within its reach. All of this gravity can attract a huge amount of matter, which bunches up around the black hole and heats up. The bunched up matter is called an accretion disk, and because of friction the gas and dust heats up, emitting infrared light. Looking at the center of the Milky Way doesn’t reveal much in visible light, but radio, infrared, and X-ray telescopes can tell us a lot about the black hole lurking there.

The Milky Way’s center is 26,000 light-years from Earth, and Sgr A* is measured to be about 14 million miles across. This means that the black hole itself would easily fit inside the orbit of Mercury. How much mass is crammed inside this relatively small space? The lower mass limit of the black hole itself is calculated to be more than 40,000 Suns. However, the radio-emitting part of Sgr A* is a bit bigger, about the size of the Earth’s orbit around the Sun (93 million miles), and weighs much, much more – 4 billion Suns.

The black hole at the center is very active, spitting out flares of gas from stars it has eaten. If you want to know more, there is a whole book written just about our very own supermassive black hole.

Sgr A* isn’t the only thing at the heart of the Milky Way. There are massive star clusters, such as the Arches,Quintuplet, and the GC star cluster. The stars in these clusters are also very bright in the X-ray part of the spectrum, as winds blowing off their surfaces collide with gas emitted from other stars in the region. The clusters are slamming into clouds of molecular gas, creating more diffuse emissions in the X-ray spectrum. These collisions may result in a higher proportion of more massive stars than low-mass ones in the Galactic center, compared to a quieter neighborhood. Here’s a longer article about the image below.

For more information on the Milky Way, listen to Episode 99 of Astronomy Cast.

Source: NASA