MILKY WAY

The galaxy we live in is a mystery to many of us. Sure, everyone knows a few facts here and there, but I can guarantee you that there is quite a bit more to know. In that spirit, Universe Today has compiled a list of links to informative articles about the Milky Way. We also thought that you might to enjoy a few quick and occasionally quirky facts about the spiral galaxy that we inhabit.

Galileo Galilei is credited with discovering the Milky Way shortly after building his first telescope.

Scientists have tried to date the Milky Way and have only been nominally successful. The closest estimate is somewhere between 10 and 13.6 billion years.

Just like many other galaxies, the center of the Milky Way is a supermassive black hole named Sagittarius A*. It is 26,000 light years away from Earth and measures 22.5 million km across. The entire galaxy is 120,000 light years in diameter.

Have you ever looked towards the heavens and wondered how many stars there are? What can be seen from Earth is only a miniscule fraction of the total number of stars in our galaxy. The best estimate that science can make is that there is somewhere between 200 and 400 billion stars in the Milky Way alone.

Those are just a few of the interesting facts, figures, and images that you will find among the links below.

How does the Galaxy in which we live look like?

It is almost certain that we will never be able to send a probe out of our Milky Way to take a snapshot, in the same way as the first satellites could do to give us striking images of planet Earth. But astronomers do not need this to imagine what our bigger home resembles. And they have a pretty good idea of it.

The Milky Way with its several hundreds of billion stars is thought to be a relatively flat disc -100,000 light-year across- with a central bulge lying in the direction of the constellation Sagittarius (The Archer) and six spiral arms. The Milky Way has most probably also a central bar made of young, bright stars.

If we can’t take pictures of the Milky Way, we may photograph others galaxies which astronomers think look similar to it. The two galaxies presented here are just two magnificient examples of barred spiral galaxies. One – Messier 83 – is seen face-on, and the other – NGC 4565 – appears edge-on. Together, they give us a nice idea of how the Milky Way may appear from outer space.

These images are based on data obtained with the twin FORS1 and FORS2 (FOcal Reducer and Spectrograph) instruments attached to two ESO’s 8.2-m Unit Telescopes of the Very Large Telescope Array located on Cerro Paranal. The data were extracted from the ESO Science Archive Facility, which contains approximately 50 Terabytes of scientific data and is, since April 1, 2005, open to the worldwide community. These invaluable data have already led to the publication of more than 1000 scientific papers. They also contains many nice examples of beautiful astronomical objects which could be the theme of as many midsummer’s dreams.

NGC 4565

The first galaxy pictured here is NGC 4565, which for obvious reasons is also called the Needle Galaxy. First spotted in 1785 by Uranus’ discoverer, Sir William Herschel (1738-1822), this is one of the most famous example of an edge-on spiral galaxy and is located some 30 million light-years away in the constellation Coma Berenices (Berenice’s Hair). It displays a bright yellowish central bulge that juts out above most impressive dust lanes.

Because it is relatively close (it is only 12 times farther away than Messier 31, the Andromeda galaxy, which is the major galaxy closest to us) and relatively large (roughly one third larger than the Milky Way), it does not fit entirely into the field of view of the FORS instrument (about 7 x 7 arcmin2).

Many background galaxies are also visible in this FORS image, giving full meaning to their nickname of “island universes”.

If our Milky Way were to resemble this one, we certainly would be proud of our home! The beautiful spiral galaxy Messier 83 is located in the southern constellation Hydra (the Water Snake) and is also known as NGC 5236 and as the Southern Pinwheel galaxy. Its distance is about 15 million light-years. Being about twice as small as the Milky Way, its size on the sky is 11×10 arcmin2.

The image show clumpy, well-defined spiral arms that are rich in young stars, while the disc reveals a complex system of intricate dust lanes. This galaxy is known to be a site of vigorous star formation.

Age of the Milky Way

Artist

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.