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Posts Tagged ‘stellar evolution’

A couple of weeks ago this news item caught my attention, and it even made it onto the BBC Radio 5 news. Astronomers have discovered what they think could be the most powerful supernova ever seen. A supernova is when a massive star explodes, and they usually outshine their host galaxy when they do so.

Incredibly, the discovery of this particular supernova was not made with a large telescope but rather by using a telephoto lens just like sports photographers use! The lens is part of a suite of telephoto lenses which scan the skies from a mountain top in Chile, looking for such exploding stars.

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One of the most powerful supernovae ever seen was actually imaged using a telephoto lens, not a telescope!

Supernovae are the product of massive stars coming to the end of their lives. Our Sun will never explode as a supernova, it is just not massive enough. But we think that if a star is about 3 times the mass of our Sun or more, then it will go through a series of nuclear reactions which culminate in it blowing itself apart. I have blogged briefly about stellar evolution, and in particular stellar death, here, but I think I need to do a more detailed blog on the latter stages of the life of high mass stars. Amongst other things, it is in supernovae that all the elements beyond carbon are created. So, they play a vital part in not only the formation of planets, but in the formation of life!

 

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The other week I saw this picture on NASA’s Astronomy Picture of the Day, namely the beautiful planetary nebula Abell 36. To be honest, Abell 36 is not a planetary nebulae with which I was familiar before seeing this photograph.



Abell 36, a beautiful planetary nebula in the constellation Virgo

Abell 36, a beautiful planetary nebula in the constellation Virgo.



It reminded me of (a) just how stunningly beautiful planetary nebulae are and (b) that it was high time I wrote a blog about planetary nebulae to explain what they are.

First of all, the name “planetary nebula” is a complete misnomer, they have nothing to do with planets. The name arrises from their appearance through 18th Century telescopes, where their fuzzy roundness made astronomers like William Herschel think that they looked like large gaseous planets such as Jupiter and Saturn. They are, in fact, the end stages of a star like the Sun and have nothing to do with gaseous planets.

As I mentioned in my blog about the Herztsprung-Russell diagram (HR diagram), massive stars end their lives in spectacular supernovae, exploding and throwing the heavier elements that they create into the interstellar medium. Stars which have initial masses of less than about 3 times the mass of the Sun, however, will not explode in a supernova but rather they end their lives as white dwarfs. It is these low-mass stars which produce planetary nebulae.

The planetary nebula phase of a low-mass star comes towards the end of its life, after it has finished burning helium in its core (converting it to carbon), and when it starts burning helium in a shell around the core. This causes the star to leave the horizontal branch of the HR diagram and ascend the red-giant branch for the second time, and as it does this it gently blows off its outer layers of gases (mainly but not exclusively hydrogen). This gas then gets ionised by the hot white dwarf which has formed at the centre, and the gases fluoresce in the spectacular display we see as a planetary nebula. The physics of their fluorescing is exactly the same physics as produces emission line spectra, which I discussed here.

One of the best known planetary nebula is the Ring Nebula, Messier 57, which is one of my favourite Messier objects. M57 is visible high in the summer sky in the Northern Hemisphere in the constellation Lyra, not too far from Vega which is a very bright and easy to find star. It is well worth trying to see M57 over the next few months if you get the chance. You cannot see it with the naked eye, but it is visible through good binoculars or a telescope. If you look at it through a large enough telescope you can even see the white dwarf at the centre, which is not the case for many planetary nebulae. A DSLR image using even a smallish telescope should show the white dwarf quite clearly.

As the caption to this image of Abell 36 on the APOD website points out, the central white dwarf of a planetary nebula is usually extremely hot, emitting far more light in the ultraviolet part of the spectrum than in the visible part. Although the central white dwarf is clearly visible in the photograph of Abell 36, it would be much brighter than the nebula if one were to view it in UV light. To this end, you have more chance of seeing the central white dwarf is you use a blue filter to block out most of the light from the nebula (which tends to be dominated by the red hydrogen-alpha line), but of course if you do this the nebula itself will appear much less bright or may even disappear. So, depending on what you want to see, choose any filter you may have accordingly. If you want to see the nebula, either don’t use a filter or use a red one, even a hydrogen-alpha one if you have one, but if you want to see the white dwarf, use a blue filter to reduce the brightness of the nebulosity.



Messier 57, the Ring Nebula. It is one of my favourite Messier objects. The white dwarf can be seen at the centre of the nebula.

Messier 57, the Ring Nebula. It is one of my favourite Messier objects. The white dwarf can be seen at the centre of the nebula.



Here is a finding chart to help you find it in the sky.


A finding chart for Messier 57 (the Ring Nebula). It can be found between the stars

A finding chart for Messier 57 (the Ring Nebula). It can be found between the stars Sulafat and Sheliak.



Finally, let me share two photographs of two of the other best known planetary nebulae, the Helix Nebula and the Eskimo Nebula. Again, both beautiful examples of planetary nebulae.



The beautiful Helix Nebula, which is in the constellation  Aquarius.

The beautiful Helix Nebula, which is in the constellation Aquarius.



The Eskimo Nebula, in the constellation Gemini.

The Eskimo Nebula, in the constellation Gemini.




Which is your favourite planetary nebula?

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Probably the most important diagram in stellar astrophysics is the Hertzprung-Russell diagram. It was arrived at independently by Danish astronomer Enjar Hertzsprung in 1911 and American astrophysicist Henry Norris Russell in 1913.

By the start of the 1910s, the Harvard College Observatory’s programme of obtaining the spectra of stars and classifying them (see my blog about that here) meant that the spectral type of a star was one of its measured properties. Another measured property was a star’s apparent magnitude, how bright it appears to be in the sky. As I’ve mentioned before, to calculate a star’s intrinsic brightness (its absolute magnitude>, we also need to know a star’s distance. But with the stellar parallax measured for about one hundred stars, and the period-luminosity relationship for Cepheid variables newly discovered by Henrietta Leavitt, in the early 1910s astronomers were beginning to have a way to convert from a star’s apparent magnitude to its absolute magnitude, and hence measure their intrinsic luminosities.



Henry Norris Russell, who was based at Princeton as an Assistant Professor when he came up with his version of the H-R diagram in 1913.

Henry Norris Russell was based at Princeton as an Assistant Professor when he came up with his version of the H-R diagram in 1913. He would spend the rest of his career at Princeton, and became one of the foremost astrophysicists in the early 20th Century.



Enjar Hertzsprung, who was based at Copenhagen when he came up with his version of the H-R diagram in 1911. He would spend most of his career at Leiden in the Netherlands.

Enjar Hertzsprung, who was based at Copenhagen when he came up with his version of the H-R diagram in 1911. He would spend most of his career at Leiden in the Netherlands.



Hertzsprung and Russell independently decided to plot the spectral type (or colour, or surface temperature; they are related) of a collection of stars against their absolute magnitudes (intrinsic luminosities). What they found was intriguing – rather than stars appearing all over the plot as they might have expected, most stars lay along a well defined band which stretched from the top left to the bottom right. Some stars were above this band, and a few were below it.



The Hertzsprung-Russell diagram is usually plotted with temperature (or spectral type, or colour) on the x-axis and absolute magnitude (intrinsic brightness) on the y-axis. Also, the x-axis is backwards with the temperature increasing going towards the left! The band which stretches from the top left to the bottom right is called the main sequence

The Hertzsprung-Russell diagram is usually plotted with temperature (or spectral type, or colour) on the x-axis and absolute magnitude (intrinsic brightness) on the y-axis. Also, the x-axis is backwards with the temperature increasing going towards the left! The band which stretches from the top left to the bottom right is called the main sequence, and all stars on the main sequence are burning hydrogen in their cores.



Clearly the concentrations of stars in different places on the H-R diagram was telling us something, but it would be a number of decades before astronomers had got it all figured out. In fact, it is one of the great triumphs of 20th Century astronomy that we went from not understanding the H-R diagram when it was first presented by Hertzsprung and Russell to having an essentially complete understanding of it by the 1950s. In the early 1910s we didn’t now that stars were mainly hydrogen, we didn’t know from where they got their energy, and we didn’t know about the life-cycle of stars.

You can think of the H-R diagram as giving the history of stellar evolution. A star like our Sun will be in different parts of the H-R diagram at different stages of its life, it is currently on the main sequence as it is burning hydrogen in its core. It has been in this part of the H-R diagram for the last 4.6 billion years, and should remain on the main sequence happily burning hydrogen in its core for about another 5-6 billion years.

Clearly, with these kind of time scales, we cannot see the evolution of an individual star. But, luckily for us, with thousands of stars visible to even the naked eye, and millions visible through telescopes, we are able to see stars at each stage of stars’ evolution, from birth to death.

All of the stars which are on the band of points which stretches from the top left to the bottom right, which we call the Main Sequence, are burning hydrogen in their cores. They are the only stars on the H-R diagram which are burning hydrogen in their cores, if a star is not burning hydrogen in its core it will not be on the main sequence. Our Sun is a main sequence star, and this stage of a star’s life is the most stable period of its evolution, and it spends some 90% of its life on the main sequence.

When the Sun formed, it would have started off as a cool nebula (cloud of gas and dust) which would have started collapsing under gravity, and the gravitational collapse converts gravitational potential energy into kinetic energy of the gas, so the nebula gets warmer. As the nebula collapses more and more the central concentration forms a proto-star, and at this point the proto-Sun would have appeared to the right of the main sequence, at a temperature of some 2000-3000 Kelvin. Its luminosity would have been comparable to the Sun’s present luminosity, or at least not much less, but this is because of the proto-star’s size, it would have been hundreds of times larger than the current Sun.

As the proto-Sun contracted still further the density and temperature in the core would have become high enough for hydrogen fusion to start. In this process, which I will describe in more detail in another blog, hydrogen is converted into helium. The Sun then settled down to a long life on the main sequence; our Sun will spend some 10 billion years in this part of its evolution.

The stars above the main sequence are red giant stars, and these are stars which have finished burning hydrogen in their cores and have started burning it in a shell about a helium core. When a star does this it swells up, becoming cooler and much brighter. It moves away from the main sequence, in a process we call “ascending the red giant branch”. Eventually the helium in the core will start to fuse, with three helium nuclei combining to form carbon, and when a star is in this phase of its evolution it sits on a part of the diagram called the horizontal branch.

For low mass stars, less than about three times the mass of the Sun, converting helium to carbon is as far as they will get in their nuclosynthesis. After they have finished on the horizontal branch, these low mass stars will blow off their outer layers and form what we call a planetary nebula, leaving a dead carbon core behind which cools over time. The name for this dead carbon core is a white dwarf

Higher mass stars, more than about three times the mass of the Sun, are able to go beyond helium burning, and can burn carbon, and sometimes oxygen, silicon and other elements all the way up to iron. However, a star will never burn iron, as iron is the most tightly bound nucleus of all the elements in the periodic table, and so such high mass tars have a sudden energy crisis and explode in a supernova. I will blog about the details low mass and high mass stellar evolution in a pair of future blogs.

So, to summarise, the H-R diagram is in a visual illustration of stellar evolution, and now we understand how stars move about on the H-R diagram during their lives, a triumph of 20th Century astrophysics.

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