The Ecology of Stars

Transcription

1 The Ecology of Stars We have been considering stars as individuals; what they are doing and what will happen to them Now we want to look at their surroundings And their births 1

2 Interstellar Matter Space is nowhere truly empty There is material the between the stars composed of gas and dust We call this material interstellar matter" Nebulas Dense patches of this interstellar matter (1000X > than normal space) show up in the form of nebulas Some of which are visible... 2

3 Some of which are not visible... So why study Nebulas? They are the material from which new stars are born They sometimes hide and distort what lies behind them Scientists love mysteries!! They are some of the most beautiful formations in all the Universe 3

4 Interstellar Gas 75% hydrogen, 25% helium, and about 1% other Basically like the Sun The gas appears in three forms: Ionized As in a bright nebula Neutral Cool, dense gas Molecular How can we find this gas? This is based on the type of gas nebula it is... Ionized Nebulae They show up as bright gas clouds The gas is bright because it is hot About 10,000 K 4

5 The gas gets its heat from nearby young stars (O & B-type) which are ionizing the gas For this reason they give off an emission line spectrum Therefore we call them Emission Nebulae The gas in a nebula is often the remnants of the gas cloud out of which the O & B stars formed Neutral Gas These do not give off light! We have two means by which to detect this neutral gas: Diffuse Absorption lines 21 cm Radio lines 5

6 Diffuse Absorption lines A few elements have absorption lines in the visible, though more often they are in the ultraviolet Remember that this means that the starlight filtered through these invisible clouds causes absorption lines to appear in their spectrum 21 cm Radio lines A 21 cm long Radio wave is emitted when an electron in the ground state of a hydrogen atom flips over and spins in the opposite direction We can map out the locations of large concentrations of neutral hydrogen in the galaxy by studying this radio wavelength 6

7 Molecular Gas These are cold, dense interstellar clouds which contain a high fraction of molecules Many organic molecules have been found in these clouds Radio map of carbon monoxide Interstellar Dust Particles of minerals and ice grains Very tiny Roughly 10-8 m in size (comparable to the wavelength of visible light) About 1% of the mass of interstellar space is in the dust grains Dusty regions of interstellar space allow radio and infrared radiation to pass through them but not visible light, ultraviolet, and X-ray radiation 7

8 How can we find this dust? Thick clouds can hide stars This is called extinction Starlight absorbed by the dust heats it up 20 K - 50 K Hot enough to produce infrared radiation Reddening Thinner dust clouds only partly dim the light Blue light is absorbed and scattered more than red light, so a star with dust in front of it looks redder Like the Sun at Sunset or Sunrise 8

9 Reflection Nebula The result of the blue scattered light is called a reflection nebula The Formation of Stars Gravity must overcome pressure for a cloud of gas to collapse For gravity to be strong we want a lot of matter in a small space, i.e. a high density. For pressure to be low we want a low temperature 9

10 The gas we see as nebulae (hot and bright) is not going to make new stars The dark patches we see around these nebulae will form the new stars The Stages of Star Formation The birth of a star follows a series of seven stages (steps): Stage 1 - Interstellar Cloud Stage 2 - Collapsing Cloud Stage 3 - Solar System Size Stage 4 - Protostar Stage 5 - Protostar Evolution Stage 6 - A Star is Born Stage 7 - Reaching the Main Sequence 10

11 Stage 1 - Interstellar Cloud A giant interstellar cloud (nebula) starts to contract, probably triggered by shock or pressure wave from nearby star As it contracts, the cloud fragments into smaller pieces Stage 2 - Collapsing Cloud Individual cloud fragments begin to collapse Once the density is high enough, there is no further fragmentation At this stage the cloud has some rotation 11

12 Stage 3 - Solar System Size The interior of the fragment has begun heating, and is about 10,000 K As it gets smaller the rotation speed increases because of the Law of Conservation of Angular Momentum As it rotates its outer regions get spun out into a disk The core of the cloud is now a protostar, and makes its first appearance on the H R diagram The core of the protostar is about 1 million Kelvin This process has taken about 100,000 years since the cloud fragmented Stage 4 - Protostar 12

13 Stage 5 - Protostar Evolution The battle between inward force of gravity and outward pressure from temperature continues As temperature increases, pressure increases So far we have... 13

14 Stage 6 - A Star is Born Pressure (temperature) continues to increase as the protostar shrinks The core reaches 10 million K Nuclear fusion begins The protostar has become a star This process has now taken about 10,000,000 years since the cloud fragmented Stage 7 - Reaching the Main Sequence The star continues to shrink and increase in temperature, until it is in equilibrium (balance between gravity and pressure) About 50 Million Years! The star has reached the main sequence and will remain there as long as it has hydrogen to fuse. 14

15 As mentioned, the average size nebula takes about 50 million years to complete this process But mass is a factor Our Sun took 30 million years to do this An O-type star takes about 160,000 years An M-type star takes about 1 billion years Time Brown Dwarfs Stars that remain at Stage 5, having lost the battle between gravity and pressure, will not go on to fuse protons in their core These are called Brown Dwarf stars Gliese 229B was the first confirmed brown dwarf ever observed. It has a surface temperature of about 900 K and is about the size of Jupiter 15

16 Solar Systems The birth of a star also appears to coincide with the birth of planets There are nine observations that lead us to this conclusion for our own Solar System: 1. Each planet is relatively isolated in space 2. The orbits of the planets are nearly circular 3. The orbits of the planets all lie in nearly the same plane 4. The direction in which the planets orbit the Sun is the same as the direction in which the Sun rotates on its axis 5. The direction in which most planets rotate on their axis is roughly the same as the direction in which the Sun rotates on its axis 6. The direction in which most of the known moons revolve about their parent planet is the same as the direction in which the planet rotates on its axis 7. Our planetary system is highly differentiated 8. Asteroids are very old and exhibit a range of properties not characteristic of the other planets or their moons 9. Comets are primitive, icy fragments that do not necessarily orbit in the ecliptic plane and reside at large distances from the Sun The large-scale architecture is too neat, and the ages of the components too uniform, to be the result of random chaotic events The overall organization points toward a single formation, an ancient but one-time event 4.6 billion years ago 16

17 At Stage 6, where the Protostar is born, the leftover outer parts of disk form a "solar nebula" Hot dust grains start to stick together and grow into "planetisimals When planetisimals get large enough gravity starts to pull them together and you get small planets (protoplanets) The Process For Our Solar System... In the inner part of the solar nebula, protoplanets grow larger by accumulating smaller objects to form the Terrestrial Planets In the outer part of the solar nebula, hydrogen and helium condense around protoplanets to form the Jovian Planets There is a lot of hydrogen and helium so the these planets can grow very large 17

18 Let's take a look from the beginning... Clusters Many stars are members of clusters of hundreds or thousands of stars There are two basic types: Globular Open 18

19 Formation of a Cluster Astronomers assume that when you see a cluster of stars they were all formed at the same time There are several reasons for believing this: High-mass stars (such as O & B-type stars) which have to be young are usually found in groups and clusters Stars ought to form in groups, since large interstellar clouds are more unstable than small ones It is theoretically very unlikely for hundreds of stars to come together to form a new cluster unless they were born that way The HR diagrams of clusters indicate that they have a definite age Globular Clusters Tightly bound Have a roughly spherical shape that is very bright in the middle Are found in the halos around galaxies Contain 100,000 1,000,000 stars in a region that is about 25 pc (81.5 ly) across Contain very little interstellar matter and no evidence of recent star formation Contain a low abundance of elements other than hydrogen and helium They are stable and are not dispersing 19

20 The Age of a Globular Cluster They lack upper mainsequence stars In fact, they contain no main-sequence stars with masses greater than about 0.8 times the mass of the Sun About billion years old Loosely bound collection of stars They have an irregular appearance Generally found in the plane of galaxies Contain stars in a region about 3-30 pc ( ly) across Open Clusters They often have interstellar matter with them They have a higher abundance of elements than Globular clusters, in some cases more than the Sun 20

21 The Age of Open Clusters Open clusters can be any age younger than globular clusters but are usually much younger, up to 100 million years The Fate of a Cluster Except for globular clusters, which can hold themselves together indefinitely, clusters eventually break up and drift apart One reason for this is differential rotation of the galaxy, which we will meet later Our Sun was probably a member of a cluster when it was young, but we cannot identify any of its siblings now 21

22 End Topic The Ecology of Stars 22