November 2015 – The Largest Stars in Our Galaxy

Heavens Above!

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in this month’s night sky. Last month, we discussed the mystery of dark energy and how it shapes our universe. In this issue, we will discover the largest stars known to exist.

Fig 1. The Tarantula Nebula is located within our nearest galactic neighbour, the Large Magellanic Cloud. It is home to some of the largest stars known . [NASA]Fig 1. The Tarantula Nebula is located within our nearest galactic neighbour, the Large Magellanic Cloud. It is home to some of the largest stars known . [NASA]

Fig 1. The Tarantula Nebula is located within our nearest galactic neighbour, the Large Magellanic Cloud. It is home to some of the largest stars known. [NASA]

When we look at the night sky, we see a vast expanse dotted with bits of light which we call stars. Our own sun is a star, albeit an ordinary one. But what about the extraordinary stars? The biggest stars? Let’s find out.

It’s a common theme in nature for things to be limited to certain sizes. For example, one wouldn’t expect to see a miniature giraffe because it’s neck would have no purpose. By the same logic, a giraffe capable of eating your ice cream cone from the top of the Eiffel Tower would be just as silly. Stars are the same way, but perhaps with some more concrete physics.

To understand why stars are the sizes that they are, we need to develop some sense of stellar astrophysics. Sounds complicated, but it’s not too bad. Stars are a battle between gravity and thermal pressure. Gravity likes things to collapse. Thermal pressure likes to push things apart. This phenomena is known as hydrostatic equilibrium and it describes precisely why stars are the sizes they are.

Fig 2. Hydrostatic Equilibrium holds stars together. The opposing forces of gravity and radiation pressure work to a careful balance.
Fig 2. Hydrostatic Equilibrium holds stars together. The opposing forces of gravity and radiation pressure work to a careful balance. [UC Berkley]

We all are familiar with gravity (if not, try jumping), but thermal pressure is a bit more esoteric. The question is this: what fuels the stars? If you did the calculations for any combustion process, be it with firewood or thermite, you won’t be able to replicate our Sun. For that, we need to look to fusion. In the early 1900’s, Einstein formulated his now-famous mass-energy equivalence principle, which allowed for a mathematical description of atomic fusion. The temperatures and pressures at the centre of the sun allow atoms to fuse together – which produces a great deal of excess energy. Thermonuclear bombs use this process to drive their explosive power. Our sun undergoes this process millions of times per second. The energy works against gravity to support the star.

Before discussing large stars, a quick look at brown dwarfs may be useful. If there is not enough mass to sufficiently elevate the core temperature then fusion will not start. Any star below 7% the mass of the Sun will not be big enough to sustain hydrostatic equilibrium. Blame gravity. This is what happened to brown dwarfs – which are technically speaking not even stars. If you added significantly more gas to their atmosphere, then at some point they may be able to fuse. But unless that happens, then they’re stellar duds.

As it turns out, the most massive stars in our galaxy face similar issues – but not from gravity. These stars have an additional force driving against gravity: radiation pressure. This added pressure stems from the fact that all electromagnetic radiation (light) exerts a pressure. While radiation pressure is fairly tame in low mass stars, it is a significant effect within the most massive ones.

But how big is too big? In 1916, the eminent astronomer Sir Arthur Eddington first derived an equation which predicts the absolute maximum luminosity a star of a given mass could have without loosing grips on its atmosphere due to radiation pressure. Later in 1924 he mathematically explained the long-existing relationship that the more massive a star, the higher its luminosity. Although using an overly-simplified approximation by today’s standards, Eddington concluded that there must be a point where the luminosity of a star exceeds its luminosity limit – and presto, a star no more.

Originally Eddington deduced a mass of about 65 times the mass of the sun. With better physical insight, the contemporary estimate is about three times greater at 150 solar masses. This generally agrees with observations too – a real treat for astronomers! However, massive stars live fast and die young. This means that they are fairly rare. Eta Carina, Peony Nebula Star, and the Pistol Star are some the largest we have yet found.

Fig 3. The Wolf-Rayet star WR124 is one of the largest known stars. The image was acquired by the Hubble Space Telescope.

Fig 3. The Wolf-Rayet star WR124 is one of the largest known stars. The image was acquired by the Hubble Space Telescope. [NASA]

Astronomers have a special class for these obese stellar giants: Wolf-Rayet stars. What differentiates a Wolf-Rayet star from many others is that its light is highly contaminated with heavy elements. Stars like our sun are hardly touched by metals in comparison. The observed abundances of nitrogen, carbon, and oxygen indicate significant turbulence and mixing. Only a few years ago the Hubble Space Telescope observed one of the more nearby Wolf-Rayet stars called WR 124, revealing a greatly extended atmosphere being blown away from the central star. It seems that these giants teeter on the edge of the Eddington’s Limit.

But what about the most massive star ever found? The Wolf-Rayet star R136a1 currently holds the title of the largest star known. Its a whopping 265 times more massive than our own sun! However, it’s not in our galaxy. Almost. It resides in the Tarantula Nebula within the Large Magellanic Cloud, a miniaturized companion galaxy of our own Milky Way.

Within a few million years, these stars will likely denote in a hypernova explosion, which is one of the most violent processes in the known universe. When they go off, we may even be able to see some of them from Earth – during the day.

Nature, it seems, doesn’t like being pushed to extremes.

Next Month: Violent Galaxies

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