* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download star
Observational astronomy wikipedia , lookup
Aries (constellation) wikipedia , lookup
Dyson sphere wikipedia , lookup
Canis Minor wikipedia , lookup
Corona Borealis wikipedia , lookup
Corona Australis wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Star of Bethlehem wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Type II supernova wikipedia , lookup
Star catalogue wikipedia , lookup
Canis Major wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Stellar kinematics wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Stellar evolution wikipedia , lookup
Stars! A star is a big ball of gas, with fusion going on at its center, held together by gravity! There are variations between stars, but by and large they’re really pretty simple things. Massive Star Sun-like Star Low-mass Star What is the most important thing about a star? MASS! The mass of a normal star almost completely determines its LUMINOSITY and TEMPERATURE! Note: “normal” star means a star that’s fusing Hydrogen into Helium in its center (we say “hydrogen burning”). HOW and WHY is that so? The mass of a star determines the pressure in its core: The more mass the star has, the higher the central pressure! The core supports the weight of the whole star! The core pressure determines the rate of fusion… MASS PRESSURE & TEMPERATURE …which in turn determines the star’s RATE OF FUSION luminosity! Usually, what we know is how bright the star looks to us here on Earth… We call this its Apparent Magnitude The Magnitude Scale Magnitudes are a way of assigning a number to a star so we know how bright it is Rigel and Betelgeuse, stars in Orion with apparent magnitudes 0.9 and 0.3 The historical magnitude scale… Greeks ordered the stars in the sky from brightest to faintest… …so brighter stars have smaller magnitudes. Magnitude Description 1st The 20 brightest stars 2nd stars less bright than the 20 brightest 3rd and so on... 4th getting dimmer each time 5th and more in each group, until 6th the dimmest stars (depending on your eyesight) Later, astronomers accepted and quantified this system. Modern measurements showed it was actually a logarithmic scale Every one magnitude corresponds to a factor of 2.51 change in brightness because (2.51)5 = 2.51 x 2.51 x 2.51 x 2.51 x 2.51 = 100 (99.626) 5 magnitudes change is 100 change in brightness Brighter = Smaller magnitudes Fainter = Bigger magnitudes Magnitudes can even be negative for really bright stuff! Object Apparent Magnitude The Sun -26.8 Full Moon -12.6 Venus (at brightest) -4.4 Sirius (brightest star) -1.5 Faintest naked eye stars 6 to 7 Faintest star visible from Earth telescopes ~25 log scale, no kidding? b1 b2 2.512 b – apparent brightness m – apparent magnitude m1 m2 b1 log m1 m2 log 2.512 b2 b1 1 m1 m2 log b2 log 2.512 The last thing to introduce is the question of mass That question can be translating into question: What are binary stars ???????? star A large ball of gas that creates and emits its own radiation. Binary Stars two balls – not necessarily gas, not necessarily emitting radiation >60% of Stars are in Binary Systems Contains two (or sometimes more) stars which orbit around their common center of mass. Importance - only when a star is in a binary system that we have the possibility of deriving its true mass. The period – watching the system for many years. The more unequal the masses are, the The distance between the two stars - if we know the distance to the system and their separation in more it shifts toward the more massive the sky. star. → the masses can be derived. T2 4 d G M1 M 2 2 3 The masses of many single stars can then be determined by extrapolations made from the observation of binaries. Visual Binaries The ideal case: Both stars can be seen directly, and their separation and relative motion can be followed directly. Visual Binary Stars the most common case: Spectroscopic Binaries Usually, binary separation d can not be measured directly because the stars are too close to each other. information from: Doppler Shifts for Binary Stars Idealized binary star system: two stars have equal masses and are in circular orbits and each star has a single spectral line at the same frequency when the stars are at rest. The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum. have patience Doppler shift → Measurement of radial velocities → Estimate of separation d → Estimate of masses Spectroscopic Binary Star Eclipsing Binaries There is the rare case when the system is turned so that we see it directly edge-on. This is called an eclipsing binary system. In the case of an eclipsing binary, we see each star pass directly in front of the other one. In these cases, the masses can be directly determined for the stars. Eclipsing Binary Star Algol known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered. Peculiar “double-dip” light curve NASA X-ray Binaries A special class of binary stars is the X-ray binaries, so-called because they emit X-rays. X-ray binaries are made up of a normal star and a collapsed star (a white dwarf, neutron star, or black hole). These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees!). An eclipsing binary, with an indication of the variation in intensity. An animation of an eclipsing binary system undergoing mass transfer. What is the defining characteristic of an eclipsing binary system? That at some point in its orbit one star eclipses the other along our line of sight. How many eclipses occur during a complete orbital cycle? (Two for this system) Add that most systems have two eclipses but not all systems. When does the large dip in the light curve occur? (When the hot blue star is eclipsed.)