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. The finite speed of light, three hundred thousand kilometers per second, has a consequence called light travel time. On the Earth, light essentially travels instantly. It takes light eight minutes to reach us from the Sun, so technically we are seeing...

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-Boltzmann Law shows that they must be physically small with sizes less than a tenth the size of the Sun, perhaps as low as one-hundredth the size of the Sun. These stars are called white dwarfs.

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 thousand times the size of the Sun. These exceptional stars are called supergiant stars. Some are hot and blue, and others are cool and red. Although in each case the color only refers to the outer nebulous atmosphere of the star, the centers are...

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 with the same temperature, or color, as the Sun that are much more luminous, up to ten thousand times the luminosity of the Sun or even more. Betelgeuse and Antares are two well known examples. Application of the Stephan-Boltzmann Law shows that these...

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 track that runs from hot, luminous, and blue stars in the upper left corner down to cool, faint, and red stars in the lower right corner. Stars with these properties are called main sequence stars. The main sequence runs across the H-R diagram, and it...

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 is seen in the H-R diagram where the full range of the diagram is only about a factor of 20 in temperature but a factor of 108 or a hundred million in luminosity. Lines of constant radius can also be represented on the H-R diagram according the...

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 with plotting spectral class for stars against their luminosity. They saw patterns in the ways stars appeared in this plot which led them towards an idea of how stars work. This is called the H-R diagram or the Hertzprung-Russell diagram, and it’s a key...

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, death, and life? Consider an analogy of an intelligent ant living in a forest who lives a very short time but observes the diversity of nature. For instance, there are tall trees and short trees, saplings, fallen logs. Some of the falling logs have been...

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 height, and their weight. If you plotted height against weight you would notice an obvious trend, height and weight are correlated. Most people would fall on a particular track in a diagram of height plotted against weight. Some combinations are never...

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. Stars also do not cool off. Energy flows continuously from the core where fusion occurs to the outer cooler regions. At every point within a stable star there’s an energy balance between two forces: the inward force of gravity and the outward...

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, and this can lead to an estimate of its mass. By direct observation the best situation is the case of a binary star where we can use the orbit to estimate the mass by an application of Kepler’s laws.

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. There are stars the same color as the Sun with 100 thousand times the Sun’s luminosity. By the Stephan-Boltzmann Law these stars must be three hundred times the size of the Sun. Conversely, there are stars the same color as the Sun with one-ten...

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 of 5,700 degrees Kelvin, and a radius of 700 thousand kilometers. The Stephan-Boltzmann Law gives a scaling that radius is proportional to the square root of luminosity and the temperature to the minus two power. This means that there are stars 106...

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 the constant of proportionality, the Stefan-Boltzmann constant, is 5.67 times 10-8. This means that a star with twice the area has twice the luminosity, twice the number of photons emitted per second, but a star with twice the temperature has 24 or...

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 parsec its size would be about a hundredth of a second of arc. Atmospheric blurring of stellar images blurs them to 50 to 100 times larger than this, so when we see stars on an astronomical image it never reflects the true size of a star, just the blurring...

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 one is known to be three times more distant, say by parallax measurement, than the more distant star must be nine times more luminous. Or, if we have two stars of equal luminosity and one appears four times fainter than we know the fainter star is two...

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 technically called the Bolometric luminosity. Since most stars emit most of their radiation in visible light, visible or visual luminosity and Bolometric luminosity are usually almost equal. However, this is not true for very cool or very hot stars.

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 angular motion across the plane of the sky. This is called the proper motion. For a typical stellar space velocity of 20 kilometers per second a star moves about 600 million kilometers in a year. This is a large amount of motion, but at a distance...

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 bunched up in the direction of motion causing a blueshift. When the source of waves is moving away from the observer the waves are stretched out causing a redshift. The typical size of a radial velocity of a star in the solar neighborhood is about ten...

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 directly measured through the parallax technique, the application of geometry. Stars are so far away that their motions are difficult to detect, and in general stellar motion is composed of two components, each of which has to be measured using a separate...

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 to all the other elements in the periodic table; for example, there are only three carbon atoms, two nitrogen atoms, and five oxygen atoms out of that 10 thousand. Contrast that with human material which of course is mostly water. Out of every 10...

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 element is determined by measuring one or more spectral features that exactly match the wavelengths of features of the element as seen in a lab. This is the idea of a spectral fingerprint, unique to each element in the periodic table. The amount of the...

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 stars which are also bluish with temperatures of 18,000 degrees Kelvin, blue-white A stars with temperatures of 10,000 Kelvin, white F stars, temperatures of about 7,000 Kelvin, yellowish-white G stars (the Sun is a G star) with temperatures of about...

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 cooler stars: O and B stars show ionized helium and neutral helium, A and F stars show mostly neutral hydrogen atoms, G stars show neutral hydrogen and some ionized calcium, K stars show neutral metal atoms of various species, and the coolest M stars show...

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 sequence of letters. For example: Old Boring Astronomers Find Great Kicks Mustily Regaling Napping Students. Or try this one: Overseas Broadcast, A Flash Godzilla Kills Mothra, Rodan Named Successor. Or perhaps this one, oven Baked Ants, Fried Gently,...