How Do Stars Appear from Earth? Magnitude: the brightness a star appears to have from Earth Apparent Magnitude depends on 2 things: (actual intrinsic brightness) The color of a star is related to its temperature: red stars (i.e. Betelgeuse) are cooler than yellow stars (i.e. our Sun), which are cooler than blue-white stars (i.e. Sirius)
H-R Diagram In the early 1900's, Ejnar Herstzprung and Henry Norris Russell independently made the discovery that the luminosity of a star is related to its. of stars fall along the Main Sequence, most of rest are either white dwarfs or giants
Relative Star Sizes Stars range in size by mass by a factor of : The smallest stars are about 1/10 the mass of the sun, the largest are 100 times the sun s mass. Luminosity is directly related to star (not star size) Where Does a Star Get It s Energy? Stars are gaseous spheres. Stars have two nuclear processes which produce energy:
Where Does a Star Get It s Energy? Fusion: atoms fuse together to make atom. Heavier elements can be made from the fusing together of larger atoms Fusion requires enormous temperatures ( of degrees)
Evolution of a star s core For most stars, lighter elements will keep fusing into heavier elements until is produced in the core
Where Does a Star Get It s Energy? Fission: when an element more lighter elements or particles Large, heavy elements are easier to split than lighter elements. Some are also less stable than others
Fusion, Fission, and Iron Light nuclei (H, He, C, O, etc.) can and release energy Heavy nuclei (U, Pu, etc.,) can and release energy Iron is midway between and is Iron will not fuse, even during supernovae explosions. This makes it a fairly common element in the universe
What holds a star together? Stars are balanced by 2 forces: : Stars are very massive and are bound together by their self-gravity increases as 1/r (r = radius) As a star contracts, it becomes more gravitationally bound. As a star expands, it becomes less gravitationally bound. Ideal Gas Law: P = ρ x T Pressure = Compressing a gas results in higher P & T Expanding a gas results in lower P & T
What holds a star together? Hydrostatic Equilibrium (when the 2 forces are ): Gravity causes the star to Gas pressure caused by the enormous heat generated through fusion and fission cause the star to As long as these 2 forces balance each other, a star will be stable
What holds a star together? Core-Envelope Structure: Since pressure increases as you go into a star, the greater the pressure and temperature will be Core = hot and dense Envelope = cooler, lower density Example: Sun Core: r = 0.25 r sun T = 15 million K Density = 150 g/cc Envelope: r = r sun T = 5800 K Density = 10-7 g/cc
Stellar Evolution Stars spend most of their lives in the (the stable region) fusing H into He. The two forces of gravitational collapse and expansion due to pressure are in balance Due to, a star will lose mass over time, causing it to expand and cool. Eventually, a star will no longer have the ability to fuse Hydrogen Once a star stops fusing H, it begins to expand, and moves off the main sequence to become a
Stellar Death Sun-Like Stars (up to 1.5 times mass of Sun) Once the outer gases have expanded far away from the core and cooled, they become a planetary nebula The white-hot now becomes visible, and for stars similar in mass to the sun, the star becomes a white dwarf Eventually, the dwarf cools, becoming first a dwarf and finally, a dwarf Red Dwarf
Stellar Death Large Stars (Mass > 1.5 mass of Sun): Star first becomes a follows Depending on star s mass: 1.5-3 x Sun s mass: Neutron star (aka ) is left behind > 3 x Sun s mass: remaining star material collapses to become a
Stellar Death More massive stars live shorter lives than smaller stars: Our Sun: will be on main sequence for years A super hot star (~20,000 K) will stay only years on the main sequence A cool red star (~3,000 K) will stay on the main sequence for as long as years Magnetar
Black stars do not gobble up everything in their paths if the Sun became a black hole, it would have the same gravity and the Earth would orbit it the same as it does now Black Holes Black holes will form with the collapse of stars that are at least 3 x the Sun s mass
Black Holes Since is proportional to 1/r, the more mass fitting inside a small radius, the more gravitational energy there will be The sun would be a black hole if its radius was compressed to If the gravitational pull exceeds the, nothing (not even light) can escape : distance from a black hole where the escape velocity exceeds the speed of light Sample Escape Velocities: -- Moon: 2.4 km/sec -- Sun: 617.5 km/sec -- Earth: 11.2 km/sec --Solar System: ~1,000 km/sec -- Betelgeuse: 107 km/sec -- Black Hole: 300,000 km/sec
How do we know what stars are made of? Electrons are located in a cloud outside the nucleus Electrons have different and can move to higher levels if given enough energy Electrons will drop to their most energy level whenever possible, releasing energy, which can be measured
How do we know what stars are made of? Energy drops in atoms are at very frequencies : Each element has a unique pattern of radiation that its electrons will emit when dropping energy levels. This is the element s fingerprint.
How do we know what stars are made of?