Star Formation in the Interstellar Medium (ISM)

StarFormation

Ancient History? Or Still Relevant? Are stars still being born today? If so, how do we identify regions of star formation? How does it happen?

Analogy: a Forest Is it still fertile? To decide that, ignore the mature trees. Look for saplings and shoots.

Identifying Stellar Nurseries Look for extremely young objects! O/B stars -- the hottest stars, on the blue end of the main sequence. (They live only short lives, must be recent arrivals) HII Regions – clouds of ionized hydrogen gas. Why? Because the abundant ultraviolet light from hot stars is what makes the gas fluoresce.

Example: ‘Rosette’ Nebula

..and more Dusty, gassy regions – the raw materials of star formation. Dusty, gassy regions – the raw materials of star formation. Reflection nebulae – newly formed stars may not be hot enough to fully ionize the gas, but can still be bright enough to light up extensive clouds of gas and dust Reflection nebulae – newly formed stars may not be hot enough to fully ionize the gas, but can still be bright enough to light up extensive clouds of gas and dust

Examples: the Trifid, the Pleiades

..and more Dark Clouds and ‘Globules’ – as the original gas cloud contracts under gravity, its density increases and for a time it can appear as a small dark cloud. Dark Clouds and ‘Globules’ – as the original gas cloud contracts under gravity, its density increases and for a time it can appear as a small dark cloud. Molecular clouds – in the cool confines of dense clouds, complex molecules can form and survive. (In the heat of stars, they would be disrupted.) So if we can detect them (using specialized radio telescopes), we deduce the presence of a big cool cloud of material, ready for gravity to cause its collapse. Molecular clouds – in the cool confines of dense clouds, complex molecules can form and survive. (In the heat of stars, they would be disrupted.) So if we can detect them (using specialized radio telescopes), we deduce the presence of a big cool cloud of material, ready for gravity to cause its collapse.

For Example:

…and more Infrared sources – as a dense cloud contracts under the influence of gravity, the forming “proto-stars” steadily warm up. They will glow vigorously at infrared wavelengths long before they are hot enough to ignite thermonuclear reactions and become actual stars Infrared sources – as a dense cloud contracts under the influence of gravity, the forming “proto-stars” steadily warm up. They will glow vigorously at infrared wavelengths long before they are hot enough to ignite thermonuclear reactions and become actual stars

Orion in Visible Light and in the InfraRed

…and yet more! Stars not yet on the main sequence – find a cluster in which the massive stars are already on the main sequence, but the low-mass stars are not yet there. Their gravity is weaker, and they collapse more slowly as they form! Stars not yet on the main sequence – find a cluster in which the massive stars are already on the main sequence, but the low-mass stars are not yet there. Their gravity is weaker, and they collapse more slowly as they form! T Tauri stars – young stars, including some called T Tauri stars, typically have strongly enhanced `solar winds’. (This provided the ‘Magic Broom’ when the solar system formed.) T Tauri stars – young stars, including some called T Tauri stars, typically have strongly enhanced `solar winds’. (This provided the ‘Magic Broom’ when the solar system formed.)

Example: NGC 2264

…and finally ‘Unbound’ systems – we sometimes find small groups (like quartets) of stars which are dynamically unstable. ‘Unbound’ systems – we sometimes find small groups (like quartets) of stars which are dynamically unstable. (Like an impossibly incompatible couple, they will inevitably separate and drift apart! The fact that they have not yet done so is evidence that the system is young.)

Example: the Trapezium..in the heart of the Orion Nebula

Indeed, Orion is An Ideal Location for Studying Star Formation!

It is Nearby, Conspicuous - and Easily Studied (have a look tonight!)

What We Need to Understand Consider a cloud of gas in space. 1. What determines whether it collapses or not under the influence of its own gravity? 2. Will it collapse to form a single big star, or break up into smaller lumps? 3. How long does the process take? What will we see?

‘Cloudy’ Interstellar Space The Interstellar Medium (ISM) consists of: a lot of low-density distributed material (mainly hydrogen and helium); plus a lot of low-density distributed material (mainly hydrogen and helium); plus here and there, denser accumulations (‘clouds’) made of the same material here and there, denser accumulations (‘clouds’) made of the same material

Not Like Earth’s Clouds! These are made of water droplets; but the surrounding atmosphere is mainly N 2 and O 2

Interstellar Space is Very Nearly a Vacuum! The densities are far lower than we can simulate in any laboratory on Earth!

A Delicate Balance In a cloud, gravity pulls the atoms together. But their random motions (the‘heat’of the cloud) provides a sustaining pressure. Still, if the cloud is massive enough (lots of gravity!) or if it is cool enough, the collapse will start.

This is Easier for Big Clouds In general, the density is so low that only a really huge cloud will collapse to from stars (unless the material can be made to cool off dramatically). Example: some Giant Molecular Clouds (GMCs) contain enough material to make that’s 100,000 – stars or more!

For Example: In Orion

Why No Superstars? Why doesn’t the cloud collapse to form a single star 100,000 times as massive as the Sun? (The biggest stars we see are only ~100 times the sun’s mass.)

Not Allowed! There is a limit to how big any star can be! Very massive stars have to be so hot (to hold themselves up against gravity) that the abundant radiant energy inside them (the light) will actually disrupt the star. The Eddington limit is about 150 times the mass of the Sun. No more masive stars can exist!

Fragmentation  Clusters The hugely massive clouds fragment into smaller pieces as they contract, forming star-sized lumps! (The physics of this is well understood.) Result: many stars form in big clusters (which may, however, later dissipate). Think of globular star clusters, for example!

What Then? As the individual “proto-star’ lumps contract, they slowly heat up. They glow first in the infrared, and only later in visible light (as they get hotter). On the other hand, they start out quite big. So they emit quite a lot of infrared and red light! But they gradually shrink in size.

Stars Above the Main Sequence - but moving towards it! Note that the lower-mass stars take longer to reach the main sequence. Note that the lower-mass stars take longer to reach the main sequence.

Hence, As We Saw:

The Full Cycle Gas  forms stars  which eventually recycle material (the massive stars explode; the low-mass stars ‘puff off’ a planetary nebula)  the replenished / enriched gas forms new stars… and so on.