#star evolution

IC 2220

Ground based telescopes are still punching above their weight, despite the dearth of recent space telescopes, and this one taken by the Gemini South telescope is certainly one that is doing so.

IC 2220 is a reflective nebula some 1,200 light years from us in the constellation of Carina.

Uncommonly, this reflective nebula is yellow, as the vast majority of them are reflecting the light of hot young blue stars, they appear blue in colour, however the star at the centre of this reflection has likely evolved past the blue stage, and has now began to bloat into a red giant.

Red Dwarf Habitability

When you look up in the night sky, you can see countless stars, and on a very dark night, you may even see the Milky Way, a cloud like path cut across the sky. 

That alone will tell you our Galaxy is huge, but what you’re seeing is but a tiny fraction of the largest and brightest stars in our Galaxy.

3/4 of all stars are Red Dwarf stars, and yet, with the naked eye, you cannot see a single one of them. Even the closest star to our sun, Proxima Centauri isn’t visible to the naked eye, never mind the others. 

A star like our own Sun is only visible up to around 32 light years away, yet those stars in the Milky Way are up to 20,000 light years away from us. 

So what we see in the night sky is a very poor reflection of what is really out there.

The other interesting thing about red dwarfs is their life span. Leaving aside white dwarfs and neutron stars (both dead star remnants), a star only exists when it’s able to fuse material sufficiently to hold back the weight of gravity upon it caused by its mass. While this material is hydrogen (the main component in stars) the star is said to be in it’s main sequence. Huge blue giant stars tear through that hydrogen stock at a blistering pace, within 100 million years or so, it will have exhausted it and bloated to a red giant, now fusing helium. Smaller but still hot white stars may make it to 1-2 billion years of age before this happens, and even smaller G type stars like our sun, will make it to 10 billion years (we are half way through the main sequence right now). Smaller K and M type stars live much longer, K type stars may live 20-70 billion years and for M types (Red Dwarfs) well .. half a 500 Billion upwards !  Meaning in the life of a Red dwarf, 50 entire lives of G type stars like our Sun will have come and gone.

Why is this important ? Well, how long does a habitable planet take to make ? The honest answer is, we really don’t know, the definition is restricted to the only one we are aware of, and it may turn out to be a outlier or a totally average habitable planet, we just don’t know. 

Even so, planets need time, they need time to coalesce after the star formation, they need time to find a stable orbit and for the lucky ones in the habitable zone where liquid water may exist, they may take time to develop the atmosphere and environment to be considered habitable. So, it would stand to reason, the longer a star lives, the more chance of such circumstances occurring ! 

Except, it’s not that simple. All of the young red dwarf stars we know of (up to 30 million years of age) have one thing in common, massive ultraviolet flares. The HAZMAT program uses Hubble to return constant data on a group of young, intermediate and old red dwarfs, and they found that the youngest stars were producing flares several times a day that we on Earth have seen only a few times since we first began recording it. 

It’s likely that any planet that forms in the habitable zone early on in the stars life, is going to get quite a bath of radiation, and may possibly lose its atmosphere as a result of it, although that isn’t entirely certain, it’s still a big possibility.

However, planets move, they can take a long time to find their final stable orbit, and it’s possible planets can be acquired too over time. 

Elements within a star

I was hunting for a stellar evolutionary graph to show the transition of elements within a star over its lifetime, and drew a bit of a blank, so being the nerd I am, I modelled the conversion in a spreadsheet and created this.

To disclaimer it, Stars produce all the elements up to the point where it can no longer fuse them, and lower mass stars can’t get to Iron (in fact, some lower mass stars can’t even fuse Helium). My simulator went from Hydrogen to Helium to Carbon to Oxygen etc etc. In reality, any combination of the atoms in the star core could be fused (up to Iron), but I wasn’t seeking academic accuracy, merely, to show how over time, the hydrogen drops, the helium picks up but then too (in larger mass stars) drops.

Main Sequence

After the Birth of a star, Hydrogen is the prominent component, in O type blue stars, this hydrogen fusing occurs at a ferocious pace, as the hydrogen falls and the helium builds the star swells and cools (as the surface area increases, although inside it gets hotter), changing colour slowly until Helium fusing takes over and the star is now a red giant, such as Betelgeuse is. Lower mass stars B,A,G and K type follow the same pattern, but take MUCH longer to go through the main sequence phase. M type stars (red dwarfs) can live for trillions of years, and the universe is only 14 billion years old at most, so none of them have ever burnt through their hydrogen yet, and nor will they for a very very long time.

Red Giant 

While Helium is the dominant fusion, more and more heavy elements are being created. For particularly large mass stars, Iron will begin to build up. The star cannot fuse Iron, it requires far more energy to fuse than the star is able to provide, so little by little, the amount of fusion going on begins to reduce, until the star reaches a critical point.

Supernova / Planetary nebula

In larger mass stars, the pressure outwards from the fusion collapses, and the pressure inwards from the mass of the star crushes what remains, the outer shell hydrogen begins to fuse in a massive explosion pushing out the remnants of the stars outer shell into space. The crush on the star core as it collapses is so extreme, the protons and electrons are crushed into neutrons and the entire mass becomes like a giant atom. Any more mass, and a black hole is created.

Smaller mass stars don’t explode in the same way, but rather eject the layers of their star off as fusion runs out, leaving a dead core (white dwarf), normally Carbon and Oxygen. This ejecta creates a planetary nebula around the white dwarf, and is often illuminated by it. 

Herzsprung-Russell Diagram

Pokemon hunters have their Pokedex, and Astronomers have their HR Diagram ! 

The chart first created in 1910 after the late victorian explosion in data of spectral lines of stars, charts the stars in terms of those spectral lines, provides classification for that (which we would see as colour) and identifies various groupings.

The main group is the snaking main sequence group, these are all the stars in their hydrogen core burning phase, with smaller lower temperature red dwarf (m type stars) at one end, and giant blue O type stars at the other. 

As stars age and the hydrogen begins to run out and helium burning takes over, the stars swell, but also drop in temperature, causing them to change colour, and become red giants. This group you can see under giants and supergiants. The giant O type stars quickly move through to red, and you can see the stages from Spica (a young blue giant) through to Rigel, Canopus and ending up as stars like Betelgeuse and Antares. These stars will eventually go supernova.

The smaller stars in the main sequence move towards the giant stage, but then throw off their outer layers, creating planetary nebula and become white dwarfs. 

As our Sun grows older and the hydrogen begins to deplete and Helium increase, it will eventually turn into a red giant. The current theory is that the inner planets will be engulfed by the expanding sun as it does. 

So stars that have already entered the red giant phase are important for study, particularly if they appear to have planets still, and even more so if close in planets appear to have somehow survived the transition to red giant. 

Nasa Observes ‘Growth Spurt’ From Newborn Protostar

Using data from orbiting observatories and ground-based facilities, an international team of astronomers has discovered an outburst from a star thought to be in the earliest phase of its development.

The eruption, scientists say, reveals a sudden accumulation of gas and dust by an exceptionally young protostar known as HOPS 383. Stars form within collapsing fragments of cold gas clouds. As the…

Nasa Observes ‘Growth Spurt’ From Newborn Protostar

Using data from orbiting observatories and ground-based facilities, an international team of astronomers has discovered an outburst from a star thought to be in the earliest phase of its development.

The eruption, scientists say, reveals a sudden accumulation of gas and dust by an exceptionally young protostar known as HOPS 383. Stars form within collapsing fragments of cold gas clouds. As the…