Stellar nurseries turn back time
The Milky Way is a great, flat spiral of gas and dust, containing billions of stars and planets, and is about 100,000 light years in diameter.
It is a spiral galaxy, very similar to billions of others, scattered throughout the universe out as far as we can reach with our telescopes. It is hard to see our galaxy from the inside; we can better see what is going on in other, nearby spirals, like Messier 51.
Messier 51, also known as M51, lies about 40 million light years from us, which means the light we are seeing has taken 40 million years to get to us, so we are seeing it as it was that long ago.
Spiral galaxies consist of a core region, with arms spiralling out from it. Among the most striking features scattered out along the spiral arms are little pinkish-red blobs. These are stellar nurseries – places where new stars are forming.
Huge clouds of gas and dust have collapsed to form new stars. Many of the young stars are hot, bright and blue, producing floods of ultraviolet light. This excites the hydrogen in the gas clouds, making it fluoresce with that pinkish-red colour.
This makes active star-forming regions easy to spot.
One of the numerous star-forming regions in our galaxy is in the sky at the moment and easy to spot.
The main ingredient in making stars is hydrogen, all of which was formed back at the beginning of the universe, almost 14 billion years ago. If stars simply formed from the spontaneous collapse of gas clouds, there would have been a period of very vigorous star formation, which would continue until the hydrogen was mostly used up.
Instead, today about 75 per cent of the matter in the universe is hydrogen.
The 13.5 or so billion years since the first stars formed has not made much of a dent in it. We would have only consumed at most 25 per cent. We need an additional ingredient to make stars.
Most of those gas clouds are in equilibrium, where the gravitational force pulling inwards is balanced by outward pressures that stop them from collapsing. To get star formation we need to disturb that balance.
One way is for a nearby ageing star to explode as a supernova. This can unbalance nearby clouds forming a rash of star formation. The radiation from new stars can also trigger additional star formation.
However, the biggest episodes of star formation arise when galaxies collide. These can trigger the birth of millions of new stars and planets in a relatively short time – a few million years.
Galactic collisions sound like good topics for the ultimate disaster movie. The impression may be underlined by the computer simulations of galactic collisions we can find on the web or see on TV.
In fact, galaxies are mostly empty space. It is very unlikely even in a headon collision for any stars to pass close enough to each other to affect them.
Our galaxy is due to collide with the Andromeda Galaxy, another spiral, in about four billion years.
Our remote descendants, if any, may notice that over a few million years, some new star forming regions might appear in range of their telescopes, and the shape of the Milky Way in the sky will change.
For instance, about 4.5 billion years ago, something caused a gas cloud to collapse, forming the Sun, planets and ultimately us. Was it a nearby new star, a supernova explosion or possibly a galactic collision? It is intriguing to look at those little pink blobs and think what is being created in them, and that we would see the same thing in every spiral galaxy, out in space as far as we can see.
Venus lies low in the sunset glow. Around 4am Jupiter is in the south, and Mars and Saturn, close together, to the East of Jupiter. The Moon will reach Last Quarter on the 8th.