November 17th: Encore: The Life of Stars – A View of Orion


Date: November 17, 2012

Title:Encore:  The Life of Stars – A View of Orion

Podcaster: L Cate Kendal


This podcast originally aired on March 31st, 2011 :

Description: In this podcast, we will take a brief look at the bright stars of Orion, and how they fit in to our understanding of how stars live and die.

Bio: L Cate Kendal is a science writer, theoretical physicist and amateur astronomer living in Scotland. She has written for the magazine Astronomy Now and does science outreach volunteer work with the Royal Observatory in Edinburgh.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


Hello, I’m L Cate Kendal and welcome to this podcast.

There can be few constellations that are better known than the constellation of Orion. He strides across our winter skies, with his shield up and his club held high dominating the view. And he is such a well known and familiar sight that it is easy to forget what an amazing sight it is. Not only that, it is the source of a number of well-known stars and nebulae. Betelgeuse, Rigel, the Orion Nebula, the Horsehead Nebula, these are all well-known in popular culture.

But today I’d like to talk about a different aspect of this constellation, not just to discuss what kind of stars give up this fine sight, but also take a brief look at the life cycle of stars, a life cycle we can see in Orion with the naked eye.

In this discussion about life cycles and ages we are inevitably going to run across some problems, as ages of stars are tricky things to measure. Our understanding of stellar life cycles comes from observations, and our observations are used to refine the models, but there is a certain circularity that can’t be avoided. The whole problem is that stars live for a very long time compared to us. We only get a instantaneous glimpse of the whole life cycle of any given star, even the Sun.

The coolest and longest lived stars will still be slowly consuming their nuclear fuel long after the Sun has stopped shining. Even the shortest lived stars have lives measured in millions of years.
So we can’t watch and wait around to see what happens and how stars change.

But a simple place to start is birth and childhood. And what better place to think about star birth than the Orion nebula, M42. It is the middle, fuzzy patch of Orion’s sword that hangs from his belt. Famous and easily viewed, it’s clearly not a star, even to the naked eye. The nebula is a billowing cloud of dust and gas, lit by the light of the young stars inside it. The most obvious grouping of young, heavy stars is called the Trapezium, sadly not visible to the naked eye. These stars are perhaps a few hundred thousand years old, just born in astronomical timescales.

The nebula is in fact part of a larger nebula, and this whole region is very intensely studied. Star formation may be fairly common in the universe, but it is not easy to see from our position on Earth, and is often obscured by clouds of dust. The Orion Nebula is, astronomically speaking, right on our doorstep, only about one thousand three hundred light years away, and contains stars in various early stages of formation.

We now look at the mature stars, although some of them don’t behave in a very mature kind of way. All but one of the seven stars that make up the main body of Orion adhere to the live fast, die young and go out with a bang model of life!

As I mentioned earlier, getting the ages of stars isn’t easy, but there are other ways that we can anticipate what might happen to a star. The best way to predict how a star is going to behave is to measure its mass.

The seven bright stars that make up the main constellation are relatively close to us, within about 1000 light years, except Alnilam, the middle of the belt, is which is a few hundred light years further away. So they are all close, and they all appear bright, and there reason for that is they are all giant stars that are very luminous. They are not representative of stars in general.

At the bottom right (in the Northern hemisphere) is the star Rigel. Although officially designated beta, it is the brightest star in the constellation. It’s also a multiple star system, but the primary is a blue-white supergiant star, at least eighteen times as heavy as the Sun. It’s only about ten million years old, compare this to the Sun, which is about four and a half billion years old, and it’s only got a few more million years left before it dies. If Rigel were at the centre of our solar system, it would fill up the space almost to Mercury’s orbit, and it’s about twice as hot as the Sun.

It makes an interesting contrast with the star at the bottom right, Saiph. This is about the same distance from us as Rigel, and it’s about the same size but distinctly less bright. Ironically, that’s because it’s hotter, with a surface temperature of over twice that of Rigel, so it emits more energy in a different part of the spectrum.

Both of these stars will almost certainly explode as supernovae.

Moving up, we come to the three stars of Orion’s Belt. In the northern hemisphere, from left to right they are Alnitak, Alnilam and Mintaka. Alnitak and Mintaka are multiple star systems dominated by supergiant stars. Alnilam is a lone blue-white supergiant, and one of the most luminous stars in the sky, almost four hundred thousand times more luminous as the Sun.

Unsurprisingly, these three stars are heavy enough to explode as supernova at the end of their lives.

Moving up once again, we come to the most sedate of the seven bright stars; the star at the top right hand side is Bellatrix, and this is a hot, giant star “only” about eight or nine times the mass of the Sun. Instead of ending its life in a massive explosion, Bellatrix will eventually die more peacefully than the others, and end up as a white dwarf.

These six stars are all going through the adulthood of their lives, but now we come to look at how they will look in old age. The final star is Betelgeuse, at the top left of Orion’s body. Betelgeuse is close to the end of its life. It to is only about 10 million years old, but it evolved and aged more rapidly than Rigel due to its larger initial mass. At the moment, it’s maybe eighteen or nineteen times the mass of the Sun. It’s a huge object, even stellar standards. If it was in our own solar system, it would stretch out past Earth and Mars to well into the asteroid belt.

Part of the reason for its huge radius is the fact that it is nearing the end of its life. It’s not a hot, active blue-white supergiant star like Rigel, it’s a red giant, one of the biggest kinds of star you can get. And rather than being hot, it’s cool, only about half the temperature of the Sun.

Being a heavy, giant star, it’s going to explode, probably sooner rather than later. If it happened tomorrow, no astronomer would really be that surprised! When it does it should be a naked eye object as see from here on earth, during the day, and at night it could be bright enough to cast shadows.

With Betelgeuse, we come almost full circle.

We’ve looked at the place where stars are born and the seven giant stars of Orion. These stars will explode in supernovae, spilling their contents out into space, making huge cloud of dust and gas, like the Orion Nebula, where the next generation of stars will be born, and the life cycle of stars will start over again. But there will be different stars were Orion once was.

I hope you have enjoyed this podcast. Thank you for listening.

End of podcast:

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  1. Encore: The Life of Stars – A View of Orion « Northern Lights Expeditions - November 17, 2012

    […] about life cycles and ages we are inevitably going to run across some problems, as ages of Source: 365 Days of Astronomy Posted in Northern Lights […]

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