Tuesday, 5 August 2014

Planets Across the HR Diagram Part 1

I've just got back from my first conference in Cambridge, "Characterizing Planetary Systems Across the HR Diagram". The conference attempted to provide an opportunity for people working on planets in different ways, like trying to find out how they formed, how they behave, what they look like and a host of other questions, to try and join up some of the gaps between their areas.

Over the next couple of posts I'll try and record some of the bits of the many talks and presentations I found the most interesting, but I should probably start by explaining the name of the conference.

A simplified Hertzsprung-Russel (HR) Diagram, a plot of all of the stars in the sky. The horizontal axis shows a star's surface temperature. The vertical axis shows how bright the star is.  Image Credit: ESO.
The Hertzprung-Russel, or HR, diagram is a plot of all of the stars on the sky according to how bright they are and what temperature they are. A star's position on the diagram can tell us a huge amount about what it is like and at what stage in its life it is.

Along the horizontal axis is the colour that the light from the star appears as, going from blue through yellow to red. Stars give of light in a particular way known as a blackbody spectrum, a consequence of which is that the colour that a star appears as is directly linked (with a few other things taken into account) to its surface temperature. This means that the horizontal axis is also a measure of a stars temperature. As shown in the picture, the temperature (confusingly) increases from right to left.

The vertical axis shows how bright a star is, the stars luminosity or magnitude. Note that this isn't a measurement of how bright the star appears to be from Earth (the apparent magnitude). This doesn't tell us what the star is like, as a dim, nearby star can have the same apparent magnitude as a bright star further away. The HR diagram measures how bright a star actually is, defined either by its magnitude at a set distance (the absolute magnitude) or by how much energy it gives out (its luminosity). The HR diagram shows here measures the luminosity of the stars as compared to the luminosity of the Sun.

The position of a star on the HR diagram is related to what stage it is in its life-cycle. Stars spend most of their lives on the wavy line going from bottom right to top left across the diagram. This is known as the Main-Sequence (MS), and it's where the Sun is now. Less massive, redder stars like red dwarfs are towards the bottom right of the MS, whilst massive, hot blue stars are near the top left.

At the end of their lives, most stars swell up into red giant stars many times bigger than the Sun. Whilst the temperature of the star doesn't change that much, the surface area and hence the luminosity of the star will increase dramatically. They therefore move up the HR diagram into the top right.      

From here around 5% of stars explode as supernovae. The rest blow off their outer layers, leaving behind a tiny, very dim but very hot white dwarf. This moves them to the bottom of the HR diagram.

All of this means that by plotting a star on the HR diagram we can immediately tell what kind of star it is, and at what stage it is in its life. This has many applications. For example we can use it to tell how old star clusters are, looking to see if the massive, shorter-lived stars in the top left of the HR diagram are missing.

So that's the HR diagram part of the conference name explained. Now for the "Characterizing Planetary Systems..." part.

In his talk on the first day, Kevin Schlaufman showed us a different version of the HR diagram:

Kevin Schlaufmann's image of the known exoplanet-hosting stars on the HR diagram. Image Credit: Schlaufmann et al 2013. 
This HR diagram shows only those stars that have been confirmed to host exoplanets. The most notable feature is how empty it is compared to the full HR diagram, showing the huge gaps in our knowledge about planets around giants, white dwarfs and the top end of the main sequence.

This is partly due to the techniques we use to search for exoplanets, which tend to be biased towards finding planets at smaller stars. But it also revels how little we know about planets in some of these areas.

The aim of the conference was to bring astronomers who worked on planets in some forms over all of the HR diagram, be that studying the formation of planets at the very beginning of a stars life, observations of the debris discs around giant stars, or the remnants of planetary systems at white dwarfs. By trying to bring all of those disparate areas together, we can hopefully begin to fill in some of the gaps in the planetary HR diagram.

Over the next couple of posts I'll try and do a whistle stop tour of some of the talks at the conference, highlighting those areas I found interesting (/understood). And then I'm off to another conference... until then, new blogs will be posted on Twitter.  

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