Stellar Radiation and Stellar Types SLHL Option E

Stellar Radiation and Stellar Types SL/HL – Option E. 2

Nuclear Fusion in Stars • As we will see in a future unit, the combination of two lighter elements to form a heavier element causes a liberation of energy • In stars, we see the conversion of hydrogens into helium. • Fusion requires extremely high temperatures on the order of 107 K

Fusion • As interstellar dust coalesces due to mutual gravitation, a sufficient enough mass may be reached to produce the gravitational force that will net the heat required for fusion • Failure to achieve critical mass, which is 80% the mass of our Sun, results not in fusion but instead in a hydrogen-rich object called a brown dwarf.

Balance is Key • The fusion will produce radiation, gamma photons and neutrinos, that will in turn collide with surrounding protons and electrons, thus transferring energy. • This radiation pressure acts to stabilize the Sun against gravitational collapse, creating a kind of equilibrium.

Luminosity • The energy radiated by a star is emitted uniformly in all directions, just as we would see with a light bulb • The total energy emitted by the star per unit time (i. e. the power) is called the Luminosity of the star, L • Our Sun has a luminosity of 3. 90 x 1026 W

Luminosity • We can measure brightness (with a CCD) and determine distance to learn to Luminosity of a star. (as you will see) • L is based on radius of a star and temperature of the star. – If r is equal, then higher temp = higher L – If temp is equal, then higher r = higher L

Apparent Brightness • By the time the energy released by a star arrives at Earth it will be spread out over a sphere of radius d. The energy received per unit time per unit area at the Earth is called the apparent brightness, b

Brightness, Apparently

Apparent Brightness Formula • Assuming two stars of equal L, the closer star has a greater brightness • We would have to know the L of a star and its brightness to determine distance, but since all stars are not equally bright or luminous, we cannot use the formula in that way.

The Stefan-Boltzmann Law • Allows for a comparison of the Luminosities of different stars – R is the radius of the Star – T is the surface temperature of the star – σ (lower-case sigma) is the Stefan-Boltzmann Constant = 5. 67 x 10 -8 Wm-2 K-4

Wien’s Law • Also known as the Displacement Law • Wien discovered an empirical relationship between the maximum value of the wavelength emitted by a black body and its temperature which can be stated as:

Wien’s Law • This law applies to the spectra of stars as well, which means that we can use Wien’s Law to find the temperature of a star based on the spectra of radiation that it emits • Given that we can measure find a star’s luminosity as well, we can actually use the S-B Law for the more practical purpose of determining the radius of a star.

Atomic Spectra and You! • Passing a sufficiently high potential through a tube of a specific gas will cause it to glow, as we see in the fluorescent lights in the classroom • Different elements produce different discrete spectra, unlike an incandescent light source which produces a continuous range of colors

Emission/Absorption • If we shine light from an incandescent source through a tube made of a particular gas, only the wavelengths that would be emitted by that gas are absorbed

Absorption Spectra Thus, we can determine the surface elements of a star based on the absorption spectrum that we see when we look at that star

Spectral Classification • Stars with similar spectra are grouped together into spectral classes, which relates directly to their surface temperatures. • Temperature influences a star’s ability to ionize or excite the atoms of its surface elements, thus impacting the spectrum we see • Regardless of class, all stars are essentially 74% H, 25% He, and 1% other

OBAFGKM! Spectral Class Approx. Temp. Range (K) Color Main Absorption Lines Example O 30000 – 50000 Blue violet Ionized Helium Mintaka B 10000 – 30000 Blue white Neutral Helium Rigel A 7500 – 10000 White Hydrogen Sirius A F 6000 – 7500 Yellow white Ionized Metals Canopus G 5000 – 6000 Yellow Ionized Calcium Sun K 3500 – 5000 Orange Neutral metals Aldebaran M 2500 – 3500 Red orange Titanium Oxide Betelgeuse

Types of Stars • Red Giants – very large, cooler temp than our sun • White Dwarfs – much smaller than the sun (earth size) but much hotter

Types of Stars • Neutron stars – have undergone gravitational collapse and are now mostly neutrons at their core • Super (duper) Novas – when a neutron star’s core cannot collapse further, the outer layers get reflected back outward causing a huge shock wave. – The star will tear apart (mostly) and send out a shock wave. This can cause a flash of brightness over 100 x greater than the whole universe!

Types of Stars • Pulsars – rotating neutron stars that emit (generally) radio frequencies. – We can detect these when the pole points at Earth • Black Holes – sufficiently massive to prevent even EM radiation from leaving the surface

Types of Stars • Binary Stars – almost half of the stars we can see are in fact two stars orbiting about a common center. Can be of different types. • Cepheid Variable – stars that have a luminosity that varies regularly over time, usually within a period of a few days. – The internal structure of the star actually causes it to vary in size and thus in L

Types of Binary Stars • Some binary systems can be resolved visually, that is we can see both stars. • Some require other methods to determine that they are binary.

Eclipsing Binaries • During the rotation of the stars, one star blocks the light from the other, thus the overall brightness varies periodically. • We can use this information to determine size, surface temperatures, orbital length, and other values for the system

Spectroscopic Binaries • By observing the spectra over time and accounting for Doppler shift, we can find the makeup of the two stars

The Hertzsprung-Russell (HR) Diagram • A plot dealing with Spectral Class, Luminosity, Temperature, and Absolute Magnitude, and star type…or at least a few of these at a time

• Notice that the Scales on the axes are nonlinear and that temperature is plotted from high to low