The Interstellar Medium No its not a space

The Interstellar Medium No, it’s not a space psychic 8 A

Goals • • What is the interstellar medium? What are dust clouds? What are nebulae? How do these lead to the formation of star? – Where do baby stars come from? 8 A

The Stuff Between Stars • Space isn’t empty. • Interstellar Medium – The gas and dust between the stars. All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice. 8 A

The Distribution • Picture the dust under your bed. – Fairly uniform thin layer – Some small clumps – Occasional big complexes • Interstellar dust and gas is the same. 8 A

Dust • Space is dirty. • Dust blocks or scatters some light. • Result: black clouds and patterns against the background sky. • But what light gets through, and what light doesn’t? 8 A

Absorption and Scattering • Q: Why are sunsets red? • Light is absorbed or scattered by objects the same size or smaller than its wavelength. • Dust grains = wavelength of blue light • Dust clouds: – Opaque to blue light, UV, X-rays – Transparent to red light, IR, radio • A: Whenever there is a lot of dust between you and the Sun, the blue light is absorbed or scattered leaving the only the red light. 8 A

Interstellar Reddening • Same thing with dust clouds in space. • Since space is full of dust, the farther away stars are, the redder they look. • Enough dust and eventually all visible light is scattered or absorbed. 8 A

Dust and IR • In a dark dust cloud: – Even though all visible light may be gone, we can still use IR. – If dust is warm, IR will show its blackbody emission. 8 A

And allows us to see dust where we wouldn’t otherwise expect it. 8 A

Scattered light • Q: But what happens to the blue light that is scattered? • A: Reflection nebulae. The Witchhead Nebula 8 A

The Trifid Nebula – copyright Jason Ware 8 A

Interstellar Gas • In lecture 2 B we talked about Kirchhoff’s laws and how they apply to hot and cool gases. • Let’s look at some hot and cool gases in space. Ha emission nebulae Copyright - Jason Ware 8 A

Horsehead Nebula – copyright Arne Henden Dust obscuring Ha emission nebula 8 A

Orion Nebula – copyright Robert Gendler • In order for the hydrogen to emit light, the atoms must be in the process of being excited. • The energy for the excitation comes from very hot stars (O and B stars) within the cloud. 8 A

Cold Dark Clouds • If dust clouds block light, then inside thick dust clouds there should be no light at all. • Without light, there is little energy. • With little energy, any gas inside is very, very cold. • Inside molecules can form. 8 A

Gravity vs. Pressure • Stars and other interstellar material are in a perpetual battle between forces pushing in (gravity) and forces pushing out (pressure). • Gravity comes from the mass of the cloud or star. • Pressure comes from the motion of the atoms or molecules. – Think of hot air balloons. – The hotter the air, the bigger the balloon. 8 A

Star Formation • Remember lecture 2 A: HOTTER COOLER • Cold interstellar clouds: No heat = no velocity = no outward pressure. Gravity wins. • Gas begins to contract. 8 A

How to Make a Star 8 A

1. The Interstellar Cloud • • Cold clouds can be tens of parsecs across. Thousands of times the mass of the Sun. Temperatures 10 – 100 K. In such a cloud: – If a star’s worth of matter should clump together in a denser region than the rest of the cloud: – Gravitational attraction will win out over their combined pressure. – The clump will begin to collapse. – The cold cloud will fragment. • Time: A few million years. 8 A

Orion Nebula – copyright Robert Gendler 8 A

2. Contracting Fragments • A fragment about the same mass as the Sun slowly contracts due to its gravity. • Now a few hundredths of a parsec across or 100 times the size of the Solar System. • Temperature still about the same. • In the center some heat begins to be retained. • Time: several tens of thousands of years. 8 A

Eagle Nebula – copyright J. Hester 8 A

3. Continued Contraction • Fragment now a gaseous sphere. • Size of the Solar System – (10, 000 x the size of the Sun) • Inner region is dense enough to be opaque to light (remember the convection zone of Sun). – Result: inner region heats up – 10, 000 K. – Outer edge still cool. • The central opaque part is called a protostar. • Mass increases as material rains down on it. • Time: 100, 000 years 8 A

• Visible and IR image of the hot protostars in the Orion Nebula. 8 A

4. Protostellar Evolution • • Gravity is still winning. The protostar is still shrinking. Size: Mercury’s orbit. The protostar is also still warming. – Core = 1, 000 K (recall core of Sun 15, 000 K) – Still not hot enough for nuclear fusion – Surface temp = 2000 – 3000 K (recall Sun = 5800 K) 8 A

The Evolutionary Track • Given the low temperature, the protostar has a low flux (very little energy is emitted per area of its surface). • The surface area is huge, however! • Result: Very high total luminosity (or absolute magnitude). 8 A

5. T Tauri Phase • Protostar still shrinks: 10 x the Sun. • Smaller size smaller surface area smaller luminosity. • Luminosity = 10 x the Sun • Still heats up: surface = 4000 K • Core temp = 5, 000 K • Violent surface activity creates strong winds that blow material away near the protostar’s surface. 8 A

8 A

6. A Star is Born • Time: 6 million years since the protostar formed (Stage 3). • Radius: 1, 000 km (Recall the Sun = 700, 000 km) • Core temp: 10, 000 K (Sun = 15, 000 K) – Surface temp = 4500 K • Fusion begins in core. • Energy released creates the pressure needed to almost counter the contraction from gravity. 8 A

7. The Main Sequence • Time: 30 million years since Stage 6. • Central temp is finally 15, 000 K • Pressure from energy of fusion balances gravity – Contraction ends • Surface temp is now 6000 K • While it took 40 – 50 million years to get here, the new star will spend the next 10 billion years as a main sequence star. 8 A

Now what? • The mass of the star that is formed will determine the rest of its life! • Recall: the more massive the star, the more pressure in the core. • The more pressure, the more fusion. • More fusion: – More energy produced – Hotter – Shorter life span 8 A

Open Clusters • These are the new stars. • Small groups of young stars. • Slowly drifting apart. Jewel Box – copyright Michael. Bessell 8 A

8 A