Star & Planet Formation

Transcription

1 Star & Planet Formation A rotating, collapsing cloud will flatten into a disk, with a protostar at the center. (Planets can form in the disk!) The collapse will continue until gravity is opposed by another force. What can stop the collapse? When the density gets high enough, atoms will collide and produce nuclear fusion. A star is born! A planet-forming disk around a protostar (artist s drawing)

2 ydrostatic Equilibrium Once a star ignites Nuclear fusion in its core, it has a source of energy for the rest of its life. Fusion produces intense heat and Pressure Thermal Pressure can resist the pull of gravity. When gravity and pressure forces balance the star is said to be in ydrostatic Equilibrium

3 Pressure Balances Gravity ot air balloons defy gravity

4 Star Formation What is produced?! Star formation produces a wide variety of stars! Only a few high-mass stars (O, B Type) are produced! The lowest mass stars (eg. M type) are the most common.! But what if something even smaller forms? If a star forms with a mass less than about 1/10 the mass of the Sun, it will not have enough pressure (in its core) to start nuclear fusion. These objects are not true stars, but are called brown dwarfs They don t produce much light, but can sometimes be seen in infrared wavelengths.

5 Brown Dwarfs: Failed Stars Brown dwarfs are smaller than stars but larger than planets. They have no long-term source of energy. So... They are very faint and hard to detect. The first brown dwarfs were discovered in 1995

6 What Determines a Star s Fate? " Stars found on the Main Sequence in the -R diagram are living : " They produce energy by fusion of ydrogen. " Stars live until their ydrogen fuel is burned up... R Diagram "...then they evolve, or age, and finally die. " Stars off the Main Seq. are dying or dead " ow do stars die?

7 A Star s Fate Depends on its Mass We will consider stars in 3 mass ranges:! Small Stars: Mass < 0.4 MSUN! Medium Stars: 0.4 MSUN < Mass < 8 MSUN! Large Stars: 8 MSUN < Mass

8 Small Stars Live Forever! Stars with Mass < 0.4 MSUN are called Red Dwarfs! They have spectral type M! They have very low luminosity (eg. 1/100 LSUN)! Like other Main Sequence stars, they fuse into e in their core.! This means they consume very slowly.! Also, convection allows all of their hydrogen to be consumed.! Even though they have small fuel supply, it lasts a long time: up to 100 billion years!! A Red M Dwarf

9 Quick Summary: Death of Medium Mass Stars ( MStar >0.4 MSun BUT MStar<8 MSun) Medium Mass stars like the Sun will eventually expand, and turn into Red Giants Their outer layers expand to become a planetary nebula All that remains is the hot core of the star: a white dwarf.

10 The Life of the Sun " The Sun is a Main Sequence (MS) Star " So, it fuses ydrogen into elium in its core. "...and has done so for 5 billion years. " It is very stable (in hydrostatic equilibrium) " Every second, the Sun transforms 4 million tons of mass into energy. " This tiny fraction of the Sun s mass slowly adds up... " Eventually, all ydrogen in the Sun s core will be gone! " What will happen?

11 The Sun Now: e e -On the Main Sequence -Core is: 70 % 30 % e e Plan A: Fuse into e In the Core BUT...After another 5 billion years, all the ydrogen in the Sun s core will be gone!!

12 The Sun in 5 billion years: e e e -Leaving the Main Sequence --Core is: 0 % 100 % e e Plan B: Fuse into e In a Shell around the core Shell fusion deposits heat in the star, causing it to expand... Becoming a Red Giant

13 Sun Today Sun 5 Billion Years From now The Sun will become so large it will swallow up Mercury and Venus!

14 The Sun Becomes a Red Giant When a star becomes a Giant star it will be more luminous. Its surface will also be cooler. It leaves the Main Sequence. The shell runs out of ydrogen and gravity shrinks the core. The elium core becomes so dense that a new nuclear reaction can take place: elium can fuse into Carbon.

15 ow Stars Make Carbon!Carbon is all around us. C-12! Pure Carbon: Graphite, Diamonds! Carbon Compounds: CO 2, C 4 (methane) are gasses, carbohydrates are in food, and hydrocarbons are fuel.! Every Carbon atom in the Universe was made in a star.! At temperatures of 100 Million Kelvins, three elium Nuclei can fuse to become 1 Carbon.!This is called the Triple Alpha Process.

16 Plan C: Carbon Three elium nuclei fuse into Carbon

17 The Sun s Last Gasp " In 5 billion years, the Sun will run out of ydrogen fuel " The dense core of the Sun will then fuse elium Nuclei producing Carbon and Oxygen. " elium Fusion is hotter than hydrogen fusion. " The extra heat makes the outer layers expand. " Until they are lost into space. " These layers will form a great cloud called a: planetary nebula

18 Planetary Nebulae Are the last gasps of dying stars elix Nebula (and are not related to planets!) (closeup view)

19 The Ring Nebula: A Planetary Nebula

20 The Cat Eye Nebula

21 Leaving the Main Sequence Giants When Main Sequence stars run out of ydrogen fuel, their Temp. and Luminosity change. They leave the Main Sequence to become Giant Stars. They are beginning to die...