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Stars surround Earth whichever way we look out. These stars all belong to one galaxy, our galaxy, the Milky Way galaxy. Let's learn more about it.
Introduction to Astronomy Series
  1. Introduction to Astronomy
  2. The Celestial Sphere - Right Ascension and Declination
  3. What is Angular Size?
  4. What is the Milky Way Galaxy?
  5. The Astronomical Magnitude Scale
  6. Sidereal Time, Civil Time and Solar Time
  7. Equinoxes and Solstices
  8. Parallax, Distance and Parsecs
  9. Luminosity and Flux of Stars
  10. Kepler's Laws of Planetary Motion
  11. What Are Lagrange Points?
  12. Glossary of Astronomy & Photographic Terms
  13. Astronomical Constants - Some Useful Constants for Astronomy

The Milky Way is a spiral galaxy. The spiral arms are delineated by their overabundance of bright young stars and gas clouds and are, at present, the major location for star formation. The entire Milky Way galaxy is situated within a spherical halo consisting of globular clusters which contain some of the oldest stars in the galaxy.

This vast collection of stars which includes the Sun, our own star, is the one in which we live. At the start of the twentieth century, we thought that the Universe consisted of just this one galaxy. In the intervening years, we have discovered billions more, which exist beyond our own. Some of the lights we see in the night sky aren't individual stars but vast collections of stars; they are distant galaxies. We estimate there are between 100-125 billion galaxies in the visible Universe. Today we know that our Galaxy does not occupy a special place in the Universe, yet it remains exceptional to us. It is our home, the galaxy we know best, and the one whose stellar objects we can see and study in detail.

Artists concept of the Milky Way galaxy

Artists concept of the Milky Way galaxy

Nick Risinger

Disc of Stars

The Milky Way Galaxy is an enormous collection of stars, and gas and dust, which together make up a roughly disc-shaped body that measures about 100,000 light years across and about 4,000 light years thick. A light-year is the distance light travels in a year and is equal to 9.46 million, million kilometres.

We know there are hundreds of billions of stars in the Galaxy but we don't know just how many. The number of stars is calculated using the function for the Galaxy's mass; the amount of material the Galaxy is made of. This is thought to be about 1,000 billion solar masses (1,000 billion Suns' Worth of material). This figure not only covers the visible stars, gas and dust, but also additional material that hasn't been found yet. This "dark matter" is thought to be as much as 90 percent of the Milky Way's total mass, and we don't know what it is made from, we only know it is there.

The total mass and our knowledge of the part of the Galaxy we live in are then used to estimate the number of stars in the Milky Way galaxy. Most astronomers agree on a minimum of about 200 billion with a total of probably about 500 billion.

Arms and a halo

Most of the Galaxy's visible material consists of stars at various stages in the stellar life cycle. There are hot, bright, newly formed stars; middle-aged stars such as the Sun; older red giants; planetary nebulae and neutron stars, as well as black holes. The remaining ten percent is interstellar medium, that's predominantly hydrogen and dust that includes material shed by older stars, and material that will make new stars. Both old and young stars are densely packed in the centre of the Galaxy where they form a bulge; beyond this is the disc. Within the disc are pronounced spiral arms of stars. There are plenty of stars in the spaces between the spiral arms, but because the stars in the arms are young and bright these shine out. The brilliant light of these hot, newly formed stars give the disc a blue-white tinge. The bulge stars, which are much older, give the centre a yellow tinge.

Galaxies are classified into one of four main types (spirals, barred spirals, elliptical, and irregulars) according to their shape. It has been known for over half a century that the Milky Way is a spiral galaxy, and within the past decade, astronomers have become increasingly convinced it is a barred spiral. This is because its spiral arms appear to radiate out from a bar-shaped bulge in the centre. Unfortunately, we cannot get a clear view of the centre, the spiral arms, or the Galaxy's overall shape as we are looking from within the system; we can't get that bird's eye view that we can of other galaxies.

The Sun and the Earth are about halfway from the Galaxy's centre on the inner edge of one of the spiral arms, Orion's arm. This and the other arms are named after the constellation in which they are most noticeable in Earth's sky. Working very roughly from the inside out, they are the Norma, Scutum-Crux, Sagittarius-Carina, Orion, and Perseus arms.

A sparsely populated spherical halo of old stars surrounds the disc. As Well as individual old stars the halo contains about 500 globular clusters, which are tightly packed spherical collections of hundreds of thousands of old stars. The halo is also believed to be the home of some of the Galaxy's missing "dark matter". All the stellar objects rotate around the Galaxy's centre. The disc objects orbit within the plane of the disc; they travel individually at about the same speed and not as a solid disc such as a record on a turntable. The Sun orbits once every 220 million years or so at about 220 kilometres per second. Stars in the Halo have more elongated orbits and these take them above, below and through the disc.

Heart of the Milky Way Galaxy

The centre of the Milky Way Galaxy is hidden from us in optical wavelengths by dense dust and gas, but other wavelengths such as radio allow us to see through such obstacles. The Galaxy's exact centre is occupied by an intense radio source that seems to have no orbital motion. Less than a decade ago this object, called Sagittarius A* (pronounced A-star) was identified as a massive black hole. It is smaller than the size of Saturn's orbit and has a mass of about three million solar masses.

How was the Milky Way born?

The moment of creation has fascinated philosophers and scientists throughout history. In the distant past, how did our Milky Way Galaxy from?

The creation of our Galaxy is something of a conundrum. "It is a big mystery in astronomy and cosmology," says Professor Bob Nichol, University of Portsmouth. "As yet, there is not a compelling theory that beats all others. We have seen galaxy evolution, but we have not yet witnessed galaxy formation, and until we do, we are going to be scratching our heads."

Artists concept of the Milky Way galaxy

Artists concept of the Milky Way galaxy

Nick Risinger

It is thought that a combination of events led to the formation of the Milky Way in its present form. First, there was a monolithic collapse. After the big bang, dark matter and ordinary matter were in equilibrium and spread evenly throughout the Universe. As the Universe expanded, regions of very slightly lower densities and very slightly higher densities, or 'overdensities', were formed. "As the Universe evolved, dark matter and gas flowed into the 'overdensities' and these became the sites of galaxy formation.

As the density increased, the ordinary matter cooled and began to form clumps, which then joined other clumps to make bigger clumps. Eventually, the cooling is enough for an overall collapse of the gas, from a roughly spherical shape, to give the disc, bulge and stellar halo. However, it is also possible that the central bulge and the outer galactic halo are not created together. In some observed galaxies, they seem to have a different origin, the outer galactic halo was the accumulation of a lot of very small things, unlike the bulge, which Was an accumulation of a small number of large things. This is Where other events come in, which are described in the cold dark matter dominated cosmologies. These show that galaxies are built up over time through the steady accretion of smaller dwarf galaxies, bringing with them gas, stars and dark matter. Other stellar matter is also continuously pulled into the gravity well of our Galaxy, thus further enlarging the galactic halo.

The more theories that are investigated the more questions are generated. A few that are currently being researched include:

  1. Which models and assumptions are correct? In order to make more accurate models and simulations, the basic assumptions about dark matter, gas physics, star formation and the feedback of energy into interstellar gas must be accurate.
  2. Where are all the dwarf galaxies and why are the stars in dwarfs not like the stars in our Galaxy? Simulations predict that there should be thousands of dwarf galaxy companions to the Way, but only a few dozen have so far been found.
  3. If the Milky Way was partly formed from the accretion of dwarfs, then the chemistry of stars in the dwarfs should resemble those in the Way, but they don't.

  4. How did the different parts of our Galaxy form? Is the thick disc just the thin disc warmed up by the interaction with dwarfs, or is it the remnants of cannibalised dwarfs, or is it something else?

Is there a Black Hole at the Centre of our Galaxy?

At the centre of the Milky Way lies an unknown compact entity. Located in the constellation Sagittarius and coinciding with an intense radio source called Sagittarius A* (pronounced A-star), it boasts a mass over 2.5 million times that of our Sun, squeezed into a region no greater than the distance between the Earth and the Sun. Unfortunately, that's where our knowledge ends. Some believe it is a supermassive black hole while others have hypothesised more exotic objects. Scientists still know little about its nature or how it formed, and solving the mystery of this supermassive object is one of the greatest challenges in cosmology today. Our best evidence for a supermassive object comes from Doppler studies of the orbits of stars around the Galactic Centre. They are rotating with a period of 5.6 days, so fast that the only explanation is the existence of a single object.

Another star in particular - S2 - appears to orbit in just 15 years, allowing scientists to deduce both the mass and volume of the central dark object. The astoundingly high density calculated has led researchers to reject simpler explanations such as dense clusters of dark objects - neutron stars, planets, star-sized black holes and so on - as these would become unstable within such a reduced region and collapse. Instead, the search is on for more exotic candidates.

Using the period plus spectral measurements of the visible companion's orbital speed leads to a calculated system mass of about 35 solar masses. The calculated mass of the dark object is 8-10 solar masses; much too massive to be a neutron star which has a limit of about 3 solar masses - hence a black hole theory.

Most in the astronomical community believe Sagittarius A* is a supermassive black hole, especially as some theories of galaxy formation indicate these reside at galactic centres, varying in size from millions to billions of solar masses. Further evidence that strengthens the case for the unseen object being a black hole is the emission of X-rays from its location, an indication of temperatures in the millions of Kelvins. This X-ray source at Sagittarius A* exhibits rapid variations, with time scales on the order of a millisecond. This suggests a source not larger than a light-millisecond or 300 km, so it is very compact. The only possibilities that we know that would place that much matter in such a small volume are black holes and neutron stars, and the consensus is that neutron stars can't be more massive than about 3 solar masses.

The formation of supermassive black holes is a subject that is still under investigation. It is still not completely clear whether they were the condensing seeds for galaxies or whether they are a result of galaxy formation. Others have speculated that Sagittarius A* may be a so-called boson star - a theoretical entity composed of exotic elementary particles that may also be candidates for dark matter. These strange stars have no surface and interact with normal matter only through gravity.

A supermassive black hole is is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

A supermassive black hole is is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

Sky & Telescope Magazine

For now, there is no definitive evidence either way but researchers hope to have more answers soon. Upcoming projects such as ALMA and the European Southern Observatory's VLT Interferometer will image the complex dynamics of our Galactic Centre in unprecedented detail, revealing its turbulent processes and perhaps even observe stars that orbit the supermassive black hole in as little as a year. Moreover, the next generation of infrared interferometers should allow astronomers to see the 'shadow' cast by the gravitational diffraction of light rays near the black hole and the effects of the black hole horizon. As there is no horizon in boson stars, this would provide an extremely undeniable signature of what lies at the centre of the Milky Way.

Tutorial Series

This post is part of the series Introduction to Astronomy. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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