Triple Stars

(Three's a crowd)

(Work in progress)






Tom Comerford

What is a triple star?

Single star

Binary star

or 'double star'

Triple star

or 'ternary star'

or even 'trinary star'

So how many triples are there?

Let's look at a sample of 10 000 stars:

The least massive star here is 0.1 M;

the most massive is 4 M

0.1 M stars

1 M stars

8 M stars

25 M stars

Single Binary
Triple Quadruple*
0.1 M 1 M 8 M 25 M
90 | 9 | 1 | 0 63 | 27 | 9 | 1 20 | 35 | 30 | 15 7 | 22 | 35 | 37
Single Binary
Triple Quadruple*
0.1 M 1 M 8 M 25 M
90 | 9 | 1 | 0 63 | 27 | 9 | 1 20 | 35 | 30 | 15 7 | 22 | 35 | 37

Beyond triples and quadruples

We know of systems that contain more than four stars:

Castor (6)
Nu Scorpii (7)

Numbers this high are rare , and their evolution is fairly similar to triples

Hierarchical triples

We call triples consisting of a close pair and a distant companion hierarchical

You can imagine these like a mobile, or a binary where one of its stars is replaced by another binary

Generally, the smaller orbit is faster, and contains the more massive stars

Just for fun, here's Castor:

Fastest orbit: 20 hours

Slowest orbit: > 15 000 years

Binary orbits

Binary orbits are fairly simple - you can describe them in just one equation

If nothing else happens to them, binary orbits are stable for ever, and do not change

This is useful for simulating them, as you don't need to bother simulating the orbit

In some binaries, we can actually watch this happen

(This is a real movie of Algol's central binary, whose period is 3 days)

Why are triples hierarchical?

The triple can't find a stable configuration Binaries sometimes form, but get disrupted...
...until the smallest star gets ejected completely