Transcript: The original spectral classification sequence formed by Annie Cannon was alphabetical based on the strength of the hydrogen absorption features, but it turns out that stellar temperature, a more fundamental quantity, has a complex and not direct relationship with the strength of the hydrogen features. And so the original sequence got reordered with time. The current spectral sequence of stars going from hotter stars to cooler stars goes O, B, A, F, G, K, M. There’s been an...

Transcript: Since hydrogen is the most abundant element in the universe and in stars, its spectral transitions are fundamental to stellar classification. The ground state is numbered n = 1, an electron in the lowest energy level it can have. States then go up, n = 2, n = 3, increasing in number at higher excitations from the ground state and closer and closer spacings until the atom is ionized. Transitions in and out of the n = 1 state appear mostly in the ultraviolet part of the spectrum...

Transcript: Over a hundred years ago dozens of women labored in the basement of the Harvard College Observatory paving the way for a modern understanding of stars. These women were paid 25 cents an hour, less than half of what a man would make for similar work, to do the painstaking and tedious work of classifying photographic stellar spectra. Large photographic plates had thousands of individual tiny spectra superimposed on them. The women observed these spectra through a magnifier glass,...

Transcript: Astronomical spectroscopy began with Newton dispersing the Sun’s light with a prism. In 1817, Joseph Fraunhofer dispersed the Sun’s light with much higher resolution and saw the spectrum crossed by narrow, dark absorption features. These features exactly corresponded to the wavelengths of lines from hydrogen in the laboratory, showing that the Sun was made of hydrogen. Fraunhofer had an interesting personal history. He was an orphan and as a child was working as an indentured...

Transcript: In 1872, Henry Draper was the first man to photograph stellar spectra. This was a huge advance on previous practice which just relied on naked eye observations or visual observations where the features in the spectrum had to be described and then transcribed or written down. Draper started on a long project to photograph all the bright stars in the night sky and classify their stellar spectra. He died before the project could be finished, but he left in his will money to...

Transcript: Distance is a fundamental stellar property. Without knowing distance it’s impossible to measure the luminosity or absolute brightness of a star, and so without measuring distance it’s impossible to know the true nature of a star seen in the sky whose flux is measured whether it’s a giant star, a main sequence star, or a dwarf. Parallax is difficult to measure from the ground. Typical image sizes from ground-based observatories are about an arcsecond or fraction of an arcsecond....

Transcript: The way astronomers observe and calibrate the apparent brightness of something is through the technique of photometry. Photometry allows astronomers to measure the number of photons per second coming from an astronomical source in some specified wavelength range or pass band that’s defined by a filter. A filter is simply a colored piece of glass sitting above the CCD detector in a telescope that isolates a narrow range of wavelength. CCD detectors are sensitive to a wide range...

Transcript: It’s amazing to think that the Sun, which dominates the daytime sky and brings warmth and life to the Earth, is fundamentally the same as the stars in the night sky. Galileo started this thinking with his observations using the telescope to show that the stars in the night sky have a huge range of apparent brightness. Perhaps the Sun is just the very brightest of these stars. In the late seventeenth century Christiaan Huygens estimated the distance to the stars by emitting...

Transcript: The first direct estimate of stellar distances used geometry. In 1838, Friedrich Bessel measured the parallax of the bright star 61-Cygni. This is the seasonal shift in the apparent position of the star on the sky relative to more distant stars as the Earth travels its orbit of the Sun. The shift was only 0.6 seconds of arc, a very small effect, which is in part why it took two hundred years of telescopic observations before parallax to any star was measured. Here, however,...

Transcript: The magnitude scale is defined in such a way as a magnitude difference of five magnitudes corresponds to a factor of a hundred in apparent brightness. Two and a half magnitude difference corresponds to a factor of 10 in apparent brightness. Lower numbers in the magnitude scale are brighter, which is of course the opposite of a scale set by the number of photons per second. Zero on the magnitude scale is defined by the bright star Vega. The magnitude scale can be illustrated by...

Transcript: Apparent magnitude or apparent brightness must be specified at a particular wavelength. Stars have different colors or different energy distributions, so the apparent brightness depends on the wavelength of observation. Traditionally, astronomy is done by eye, and the detector was the visual detector which is the wavelength sensitivity of the human eye peaking somewhere in the green part of the visual spectrum. This is called visual apparent brightness or visual magnitude. ...

Transcript: Apparent Brightness in astronomy is the number of photons per second collected at the Earth from an astronomical source. It depends on three things: First, the collecting area of the device used to observe the source of light. In the case of a telescope, the aperture of collecting area is much larger than the eye, so the apparent brightness is greater. It depends on the distance to the source; apparent brightness varies according to the inverse square law. If the source is...

Transcript: We can use relative brightness to show how bright various objects in the night sky are compared to the limits of technologies we use to observe the sky. In units where Vega, the bright star, is one unit of apparent brightness, the Sun is 40 billion times brighter. The full moon is 100 thousand times brighter than Vega, and for reference a 100 watt light bulb at a distance of 100 meters is 27,700 times brighter than Vega. Venus at its brightest is about 60 times brighter than...

Transcript: The apparent brightness of the Sun is a factor of 1010 or 10 billion times brighter than the brightest stars in the night sky like Vega, or Canopus, or Sirius. If we assume that the Sun and the stars are intrinsically the same type of object, that is they emit the same number of photons per second, we can use the inverse square law to say what the relative distance is to the stars and the Sun is. It must be a factor of the square-root of 1010 or 105. The stars are therefore...

Transcript: Apparent brightness does not express a star or other source of light’s true energy output. Astronomers are more interested in absolute brightness or equivalently absolute magnitude or luminosity. For example, we can consider a situation where the apparent brightness of a 100 watt light bulb at a distance of 100 meters is actually the same as the apparent brightness of a dim, 1 watt nightlight at a distance of 10 meters, and both of those are the same at apparent brightness as an...

Transcript: A parsec is a distance unit appropriate to the study of stars. It’s the distance that produces a parallax shift of one arcsecond. In other words, it’s the distance where the angle subtended by the star as seen from the Earth’s orbit six months apart, spanning the orbit, is one second of arc. One parsec equals 3.26 lightyears, so a parsec is slightly larger than a lightyear. Astronomers also use multiples of the parsec, a thousand parsecs, which is a kiloparsec, and a million...

Transcript: The vast distances to the nearest stars encourage astronomers to use a new unit of distance. Whereas meters and kilometers work well on the Earth, and the astronomical unit is the appropriate unit for scales within the solar system, the distance scale to the stars is given as a lightyear. A lightyear is the distance that light travels in one year. It’s equal to about 6-million-million miles or 1016 meters. It’s defined as the speed of light, three hundred thousand kilometers...

Transcript: Nearly 2,000 years ago Ptolemy's Almagest, a compendium of astronomical information, contained catalogs of star names. Ancient knowledge was brought to Europe by Arab astronomers who gave names to many of the brightest stars in the sky. The Arab article is al and so we have Algol, Aldebaran, Altair, Alcor, and others. Other stars were named from myths and legends and are often given names associated with the constellation in which they reside. Thus, Alpha Centauri is the first...

Transcript: Nearly 2,000 years ago Ptolemy's Almagest, a compendium of astronomical information, contained catalogs of star names. Ancient knowledge was brought to Europe by Arab astronomers who gave names to many of the brightest stars in the sky. The Arab article is al and so we have Algol, Aldebaran, Altair, Alcor, and others. Other stars were named from myths and legends and are often given names associated with the constellation in which they reside. Thus, Alpha Centauri is the first...

Transcript: Almost all of the world’s energy comes from non-renewable sources or fossil fuels. These energy sources took 300 million years to aggregate on the Earth, originating with solar energy and living organisms. We have significantly depleted them in only one hundred years. Modern energy usage is very inefficient; the United States wastes energy equivalent to $300 billion a year. The standard modes of energy use are also inefficient. An incandescent light is only five percent...

Transcript: The solar constant is the amount of Sun’s radiation that reaches the Earth. Assuming the atmosphere to be perfectly transparent, 1,370 joules reach every square meter of the Earth surface every second. Variations in the solar constant are subtle, only about 0.1 percent per year. However, they can be larger in percentage terms at regions beyond the visible spectrum in ultraviolet rays or x-rays. In particular, in the ultraviolet ozone in the upper atmosphere can affect the...

Transcript: The Sun is the source of all the Earth’s energy, and life could not exist without the Sun. In addition, there’s growing evidence that long term variations in the Sun’s output profoundly affect the Earth’s climate. For example, in the period from 1645 to 1715 sunspots were at a generic minimum called the Maunder Minimum. And in Europe an ice age was experienced with cold temperatures, and this is confirmed by tree rings from the time. From 1540 to the present day there have...