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Jeaunace Sandrae A. Foronda Astronomy

IV-31 BSE General Science Prof. Shila Rose Sia

Star Spectra and Star Sizes

Star Spectra

The Balmer Thermometer

-Balmer lines are the spectral lines of hydrogen at wavelengths visible to the human eye.-The strength of the Balmer lines depends on the temperature of the stars surface layers. Both hot andcool stars have weak Balmer lines, but medium-temperature stars have strong Balmer lines.-If a star is cool, there are few violent collisions between atoms to excite the electrons, so the electrons ofmost atoms are in the ground state, not the second level.-In the surface layers of hot stars, on the other hand, there are many violent collisions between atoms.-In stars of an intermediate temperature, roughly 10,000 K, the collisions are just right to excite largenumbers of electrons into the second energy level.

-The key to finding temperatures from stellar spectra; a) star with Balmer lines of a certain strength mighthave either of two temperatures, one high and one low, b) the temperature at which the lines reach theirmaximum strength differs for each element, c)adding a number of chemical elements helps for finding thetemperatures of stars.

Temperature Spectral Classification

- During the 1890s astronomers at Harvard Observatory invented the first widely used system forclassifying stellar spectra. One of those scientists, Annie J. Cannon, personally inspected the spectra ofover 250,000 stars. Spectra were first classified into groups labeled A through Q, but some of thosegroups were later dropped, merged with others, or reordered. The final classification scheme includesseven major temperature spectral classes, or types, still used today: O, B, A, F, G, K, M.*- The sequence of spectral types, called the spectral sequence, is a temperature sequence. The O stars

are the hottest, and temperature decreases along the sequence to the M stars, the coolest.- The hydrogen Balmer lines are strongest in A stars that have middle-range temperatures, weak in hotterstars (O and B), and weak in cooler stars (F through M).- The study of spectral types is more than a century old, but astronomers continue to discover new types.The L dwarfs, found in 1998, are cooler and fainter than M stars. They are understood to be objectssmaller than stars but larger than planets called brown dwarfs.- The spectra of M stars contain bands produced by metal oxides such as titanium oxide(TiO), but L dwarf spectra contain bands produced by molecules such as iron hydride (FeH). The Tdwarfs, discovered in 2000, are an even cooler and fainter type of brown dwarf than L dwarfs.

Luminosity, Radius and Temperature

- A stars luminosity is affected by two factors; surface area and temperature.- Luminosity: L= 4R

2T

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Star Sizes

The H-R Diagram

- The HertzsprungRussell (HR) diagram, named after its originators, Netherlands astronomer EjnarHertzsprung and U.S. astronomer Henry Norris Russell, is a graph that separates the effects oftemperature and surface area on stellar luminosities and enables astronomers to sort and classify starsaccording to their sizes.

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- The HR diagram is a graph with luminosity on the vertical axis and temperature on the horizontal axis.A star is represented by a point on the graph that marks its luminosity and its temperature.

Giants, Supergiants, and Dwarfs

-The main sequence is the region of the HR diagram running from upper left to lower right. It includesroughly 80 percent of all stars.- In the HR diagram, some cool stars lie above the main sequence. Although they are cool, they areluminous, and that must mean they are larger and have more surface area than main sequence stars ofthe same temperature. These are called giant stars, and they are roughly 10 to 100 times larger than thesun.- There are even supergiant stars at the top of the HR diagram that are over a thousand times the sunsdiameter.- At the bottom of the HR diagram lie the economy models, stars that are very low in luminosity becausethey are very small. At the bottom end of the main sequence, the red dwarfs are not only small, they arealso cool, and that gives them low luminosities.In contrast, the white dwarfs lie in the lower left of the HR diagram and are lower in luminosity than youwould expect, given their high temperatures.

Luminosity Spectral Classification

- A stars spectrumis used to determine whether it is a main-sequence star, a giant, or a supergiant. Thelarger a star is, the less dense its atmosphere is, and that affects the widths of spectral lines.- Atoms collide often in a dense gas, their energy levels become distorted, and their spectral lines arebroadened.- In the spectrum of a main-sequence star, the Balmer lines are broad because the stars atmosphere isdense and the hydrogen atoms collide often. In the spectrum of a giant star, the lines are narrowerbecause the giant stars atmosphere is less dense, and the hydrogen atoms collide less often. In thespectrum of a supergiant star, the Balmer lines are very narrow.- Size categories derived from spectra are called luminosity classes because the size of the star is thedominating factor in determining luminosity.- The luminosity classes are represented by the Roman numerals I for supergiants through V for main-sequence stars, with supergiants further subdivided into types Ia and Ib, as follows: Ia Luminous

supergiant, Ib Supergiant, II Luminous giant, III Giant, IV Subgiant, V Main sequence.

Spectroscopic Parallax

- Astronomers can measure the stellar parallax of nearby stars, but most stars are too distant to havemeasurable parallaxes. These distances can be estimated from the stars spectral type, luminosity class,and apparent magnitude in a process called spectroscopic parallax.- Spectroscopic parallax relies on the location of the star in the HR diagram. If you record the spectrumof a star, you can determine its spectral class, and that tells you its horizontal location in the HR diagram.You can also determine its luminosity class by looking at the widths of its spectral lines, and that allowsyou to estimate the stars vertical location in the diagram. Once you plot the point that representsthe starin the HR diagram, you can read off its absolute magnitude. As you learned earlier in this chapter, youcan find the distance to a star by comparing its apparent and absolute magnitudes.