14. Stars 1
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Transcript: Since light has a finite speed, three hundred thousand kilometers per second, there’s an inevitable consequence called light travel time. In terrestrial environments light essentially travels instantly or appears to travel fast. T
Transcript: Some stars in the sky, somewhat hotter than the Sun with temperatures of 5 thousand to 10 thousand Kelvin, have very low luminosities in the range of one-hundredth to one-thousandth the Sun’s luminosity. Application of the Stephan-
Transcript: Certain rare stars in the sky with either red or blue colors are extremely luminous, up to a million times the luminosity of the Sun. Application of the Stephan-Boltzmann Law shows that their sizes must be in the range of ten to a
Transcript: A cool main sequence star with a temperature of about three thousand Kelvin lies on the main sequence with a luminosity of about a hundredth the luminosity of the Sun and a size about a quarter the Sun’s size, but there are stars wi
Transcript: As was first seen nearly a hundred years ago, when luminosities and effective temperatures are gathered for hundreds of stars near the Sun, the result is not a scatter plot. Most stars in the H-R diagram lie on a diagonal line or t
Transcript: The H-R diagram is a plot of spectral class, or equivalently effective temperature, against stellar luminosity. The Stephan-Boltzmann Law tells us that luminosity goes as a high power of the temperature, the fourth power, and this
Transcript: As a way of exploring stellar properties and understanding how stars work, in the early twentieth century two astronomers, the Danish astronomer Ejnar Hertzsprung and the American astronomer and Henry Norris Russell, experimented wi
Transcript: A human lifetime is a blink of an eye compared to the age of the stars which can be hundreds of millions or billions of years. So how is it possible for astronomers to understand the life process of a star, to see their birth, deat
Transcript: Classification is often an important first step towards physical understanding. Imagine you lived in a small town and did a survey where you gathered information on every inhabitant, three pieces of information: their age, their he
Transcript: Stars are stable. For most of their lives, fusion provides the energy source. Even though the Sun and other stars are fusing hydrogen into helium, it does not mean that they are bombs. The Sun will be stable for billions of years
Transcript: Mass is a fundamental property of a star, but it can be difficult to measure. It’s a question of how do you weigh a star in empty space? Typically astronomers make a model of the star based on the knowledge of its energy source, a
Transcript: The Stephan-Boltzmann Law allows us to understand the state of stars with the same spectral type as the Sun but with very different luminosities. In this case the scaling reduces to radius going as the square root of luminosity. T
Transcript: The Stephan-Boltzmann Law allows us to estimate the size range of stars like the Sun that get their energy from fusion of hydrogen into helium. As a reference, the Sun has a luminosity of 3.8 times 1026 watts, a surface temperature
Transcript: The Stephan-Boltzmann Law says that the luminosity of a star is proportional to its surface area and the fourth power of the temperature. If the luminosity is in watts, the radius is in meters, the temperature is in Kelvins, then t
Transcript: The size of a star is a fundamental quantity, but it’s very hard to measure because stars are so far away. The Sun, our nearest star, is half a degree across on the plane of the sky, but if we move the Sun to a distance of one pars
Transcript: Luminosity, distance, and apparent brightness are all related by the inverse square law of light. If we measure any two of these quantities we can estimate the third. For example, if two stars have the same apparent brightness but
Transcript: Stellar luminosity is a fundamental property of stars. It’s the amount of energy radiated each second. Absolute brightness is another word for this. Really we’re talking about the energy radiated at all wavelengths which is techn
Transcript: The component of a star’s motion on the plane of the sky is called the tangential velocity, and it’s typically harder to measure than a radial velocity. We need the distance to the star, typically given by parallax, and the rate of
Transcript: The component of a star’s velocity to and away from the observer is called the radial velocity, and it’s measured using the Doppler Effect. In the Doppler Effect, when a source of waves is moving towards the observer the waves are
Transcript: The branch of astronomy that deals with the positions and motions of stars is called astrometry. The positions of stars are measured by taking images of the sky. We also need to know a stars distance, typically, which is most dire
Transcript: Most stars are very different in chemical composition from you, or I, or the material on the Earth. The Sun for example, of every 10 thousand atoms has 74 hundred hydrogen atoms, 24 hundred helium atoms, and 150 or so corresponding
Transcript: Spectroscopy is the key to chemical composition to determining what a star is actually made of. There are two issues. One is detecting the presence of an element, and the second is the amount of that element. The presence of an e
Transcript: The sequence of stellar spectral classes is also a sequence of photospheric temperature. Going through the stellar sequence, we have O stars with temperatures of 30,000 Kelvin, they are white hot or even blue as seen in the sky, B
Transcript: The spectral lines that appear in a stellar sequence depend on temperature. Helium takes a larger temperature to ionize than hydrogen, and this effects the visibility of helium compared to hydrogen. Going from hotter stars to cool
Transcript: The sequence of stellar spectral classes is another example of the historical baggage that astronomers carry around. It’s hard to remember, so generations of students have dreamt up mnemonics to help them remember the unusual seque
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