Blue Stragglers: Clinging to life?

Time to jump back into Astronomy! Stars are, in my eyes, the true staple of astronomical wonder. When we look up at the sky, what’s the first thing that strikes us? Stars. And yet when it comes to public knowledge and discussion, stars are probably one of the least talked about (whereas more intense topics, like black holes, are discussed more often). Well, we know a lot of awesome things about stars, and astronomers have encountered some fascinating problems when studying these big, brilliant, beautiful objects.

One thing that needs to be known is that stars are not all the same: in fact, like human beings, they come in all colors, shapes, and sizes. It may be hard to tell with the naked eye, but next time you look up at Orion, notice that the star in the top left corner looks a lot different than the others: it’s red! This star, called Betelgeuse (yep, you read that correctly) is actually a red giant, a star in its final stages of life. This is just one example of how even without a telescope, you can observe how different some stars are.

All stars, regardless of their size, live out the majority of their life on what’s called the “Main Sequence.” A star’s size and mass have a huge effect on how it will evolve over time, including how long it stays on the Main Sequence, and how it ends its “life.” The largest stars, which are blue in color, live on the order of millions of years; stars in the middle, like the Sun, live on the order of billions of years; stars at the lowest end, red in color, can live up to a trillion years or more (for those keeping score, our Universe is only about 13.6 billion years old, so none of the smallest stars have ever died!). One of the most important graphs in Astronomy is the Hertzsprung-Russell (H-R) Diagram, shown below:

There’s a lot going on in this diagram, but try not to get intimidated. On the y-axis is the luminosity, or brightness of a star, compared to the sun’s brightness (the higher up, the brighter the star). The x-axis is temperature in Kelvin (don’t worry about the unit, just know that the farther left, the hotter the star). As you can see, the main sequence looks a lot like a straight line, and is where stars stay for most of their life. As they get older, they will move to the giant or supergiant part of the graph, and end up either in the white dwarf part of the graph after they die, or off the chart entirely. Keep this behavior in mind for the rest of this post!

One of the more interesting fields in Astronomy is the study of star clusters. Star clusters are large groups of stars which have formed together and whose mutual gravity keeps them close to each other. Because the stars all formed at the same time, we can say that they are all the same age! Therefore, we can age the cluster by placing all of the stars in the cluster onto an H-R diagram! Because the largest, brightest stars die out first, you can age the cluster by seeing the largest stars that have NOT yet died. A little morbid, but very effective.

One of the more interesting unknowns in this field are a group of stars called blue stragglers. As I explained before, very large, blue stars have very short lifetimes, dying very quickly compared to smaller stars. However, in many older star clusters (often several billion years old) we observe these blue stars that should have died out long ago. These stars are called blue stragglers, and it’s not yet known exactly why they exist!

The most likely theory (in my opinion) is that these stars are made through stellar collisions! In these older star clusters, you end up with regions of space with a lot of stars nearby. In such a densely packed environment, it seems plausible that two stars may collide and/or merge together, creating these blue stragglers who appear younger than the surrounding cluster.

There are other models which have been proposed to explain blue stragglers, but the jury is still out. This is just one wondrous example of the types of mysteries astronomers devote their time deciphering! There’s still so much to learn about the Universe, and as a fellow scientist, it’s this kind of meaty problem that I would love to sink my teeth into. If you have any questions or comments, feel free to leave them on this entry! Until next time!