Jessica and Alex, of St Marys School, Mosgiel, asks :-

Why are baby stars blue?

Karen Pollard, an astronomer at the University of Canterbury, responded.

Some baby stars are blue, but some can be red (and other colours) too!

Stars are born from giant, swirling clouds of gas and dust. These clouds are huge and therefore many stars can be created from the same cloud at the same time. This means that stars are generally born in groups - we call them "star clusters".

Stars, like people, are born with various "masses" (birth weights!). A few stars in a new cluster are very big (massive), bright and hot and these stars are also the bluest in colour. However, most of the stars in a cluster are much smaller. The medium-mass stars are cooler than the big, blue stars so are yellow in colour (like our Sun), and the very smallest, coolest stars are red. When you see a new cluster of stars, it tends to look blue because the brightest stars are blue.

When stars are first born (and they first start to shine due to nuclear reactions deep inside the star), there is still a lot of gas and dust around them. The light from the young stars can reflect off the nearby gas and dust and this scattered light generally looks blue. This is the same reason why the sky looks blue, because the sunlight is scattered off the molecules in the atmosphere and blue light is scattered the most. One of the most well-known star clusters which you may have seen, or seen photos of, is called "Matariki" or "The Pleiades". Matariki is a cluster of young stars that looks bluish because the blue light is scattered more by the wispy clouds of gas and dust surrounding the baby stars.

So, baby stars can be a range of colours. A young cluster of stars generally looks blue because the biggest, brightest stars are bluish in colour and there can often still be gas and dust around which scatters blue light more.