Stars emit a lot of energy- the source for this energy is the fusion of hydrogen into helium- it is a NUCLEAR reaction, and is often referred to as ‘hydrogen burning’. NOTE: not a combustion reaction.
The mass of the products is less than the mass of the reactants- it can be calculated (using Einstein’s equation) that the Sun is losing mass at the rate of 4 x 10^9 kg/s, and eventually all this energy is radiated from the surface.
E2.2- Explain that in a stable star (like the Sun) there is an equilibrium between radiated pressure and gravitational pressure.
As the star radiates energy, there is a force exerted outwards (radiation force), and as this is applied over an area, it is known as the radiation pressure. If there was no counter force, the star would lose its outer layers- thus there must be an equal and opposite force. This force is the gravitational force, towards the centre of the star- gravitational pressure exerted in the opposite direction; these pressures are in equilibrium- EQUAL to each other.
E2.3- Define the LUMINOSITY of a star.
Luminosity (L): the total power radiated by a star (Watts) - OR- the energy emitted per unit time (J/s)
E2.4- Define the APPARENT BRIGHTNESS and state how it is measured.
Apparent brightness: the power INCIDENT on Earth, perpendicular to unit area.
Basically- it is related to the luminosity by the distance the star is from the Earth. If two stars have the same luminosity, the one CLOSER to the Earth will have a greater apparent brightness.
To measure the apparent brightness, a CCD (Charged-couple device) is used- it uses the PHOTO ELECTRIC effect.
E2.5: Apply the Stefan-Boltzmann law to compare the luminosities of different stars.
The Stefan- Boltzmann law relates the energy radiated per unit time to the temperature of a black body. A black body is a perfect absorber and emitter of radiation. The energy emitted per unit time is known as the POWER and in this case, also as the LUMINOSITY.
The law states that the TOTAL ENERGY RADIATED PER UNIT SURFACE AREA IN UNIT TIME FROM A BLACK BODY IS DIRECTLY PROPORTIONAL TO THE FOURTH POWER OF THE KELVIN TEMPERATURE OF THE BODY.
E2.6- State Wien's (displacement) law and apply it to explain the connection between the colour and temperature of stars.
Hot objects emit electromagnetic radiation, and as the temperature reaches 1000 degrees Celsius, some of the radiation will be in the visible region of the spectrum. Stars can be assumed to be PERFECT EMITTERS.
http://astro.unl.edu/classaction/animations/light/bbexplorer.html
These graphs show the spectrum of radiation from black-body emitters at different temperatures. A hot object emits radiation across a broad range, and there is a peak in intensity at a particular wavelength. THE HOTTER THE BODY, the HIGHER the intensity peak, and the SHORTER the wavelength.
The peak wavelength, at which the maximum amount of radiation is emitted, is related to the surface temperature by WIEN'S DISPLACEMENT LAW.
It states that the PEAK wavelength of the emission of a black-body is inversely proportion to its temperature.
If the energy emitted is analysed over a range of wavelengths, and the peak wavelength is determined, then the surface temperature of the star can be determined.
As temperature increases, the total energy emitted increases; this can be seen from the graphs as the area under the curves increases- it is not a linear relationship as the area does not increase evenly.
Stars with high surface temperatures will emit radiation over the full range of visible frequencies, and so they will appear to be white. From Wien's law, we know that stars with lower surface temperatures will emit more light of a higher wavelength, and will thus appear to be red.
QUESTIONS:
5) Wavelength= (2.90 x 10^-3)/T
500 x 10^-9 = (2.90 x 10^-3) / T
T= 5800 K
6) Radius= 3.1 x 10^11
Area= 4 x pi x r^2
=1.2 x 10^24 m
L= 5.67 x 10^-8 x A x T^4
= 5.67 x 10^-8 x 1.2 x 10^24 x (2800)^4
=4.2 x 10^30 Watts
E2.7- Explain how atomic spectra may be used to deduce physical and chemical data for stars.
The radiation from stars is not a perfect continuous spectrum- there are particular wavelengths that are missing.
This is the absorption spectrum for our SUN:
The missing wavelengths correspond to the absorption spectrum of a number of elements.
The absorption is taking place in the outer layers of the star. This means that we have a way of telling what elements exist in the star- at least in its outer layers.
Thus, the chemical composition of a star can be found by analyzing the absorption spectrum.
E2.8- Describe the overall classification system of spectral classes.
Different stars give out different spectra of light- this allows us to classify stars by their SPECTRAL CLASS. Stars that emit the same type of spectrum are allocated to the same spectral class- the different letters correspond to different surface temperatures.
The spectral classes are in order of DECREASING surface temperature-
Orange
Bananas
Are
Filthy
Go
Kill
Monkeys
E2.9- Describe the different types of stars.
SINGLE STARS:
-Red giants: These stars are LARGE and RED- as they are red, they are comparatively COOL. They are one of the later possible stages for a star (our sun will eventually become a RED GIANT). The source of energy is FUSION, but of larger elements than hydrogen.
-White dwarfs: These stars are SMALL and WHITE- as they are white, they are comparatively HOT. They are on e of the final stages for some stars. Fusion is no longer taking place, and a white dwarf is just a hot remnant that is cooling down. Eventually, it will stop giving out light when it becomes sufficiently cool, and will then be known as a BROWN DWARF.
-Cepheid variables: These stars are unstable- they have VARIATION in brightness, and thus a varying luminosity. This is thought to be due to an oscillation in the size of the star. They are RARE, but useful because there is a link between the period of brightness variation and their average luminosity- it can help calculate the distance to some galaxies.
E2.9- Describe the different types of stars.
SINGLE STARS:
-Red giants: These stars are LARGE and RED- as they are red, they are comparatively COOL. They are one of the later possible stages for a star (our sun will eventually become a RED GIANT). The source of energy is FUSION, but of larger elements than hydrogen.
-White dwarfs: These stars are SMALL and WHITE- as they are white, they are comparatively HOT. They are on e of the final stages for some stars. Fusion is no longer taking place, and a white dwarf is just a hot remnant that is cooling down. Eventually, it will stop giving out light when it becomes sufficiently cool, and will then be known as a BROWN DWARF.
-Cepheid variables: These stars are unstable- they have VARIATION in brightness, and thus a varying luminosity. This is thought to be due to an oscillation in the size of the star. They are RARE, but useful because there is a link between the period of brightness variation and their average luminosity- it can help calculate the distance to some galaxies.
Binary stars rotate about their common centre of mass.
E2.10- Discuss the characteristics of spectroscopic and eclipsing binary stars.
Binary stars that can be seen with the naked eye or with a telescope are called VISUAL BINARIES. When the stars are further away, or closer together, visual resolution is more difficult.
A spectroscopic binary star is identified from the analysis of the spectrum of light emitted from the star. Over time, the wavelengths show a periodic shift, or splitting in frequency. This means that as the stars move around their common centre of mass, one star will be approaching as the other is receding.
When the star APPROACHES us, it is BLUE SHIFTED- the wavelengths are smaller so the spectral lines are shifted towards the blue side of the spectrum. As the star RECEDES, it is RED-SHIFTED- longer wavelengths therefore moves to the red-side of the spectrum.
Eclipsing binary stars how a periodic variation in the brightness of light emitted from the star system, This occurs because during their rotation, one star periodically obscures or eclipses the other.
2.11- Identify the general regions of star types on a Hertzberg-Russell (HR) diagram.
For most stars, there is a relationship between surface temperature and luminosity.
The dots on the diagram represent stars, and the scales are not linear. The temperature scale runs BACKWARDS (high temps on the left. Stars increase in size as we move up the main sequence.
The absolute magnitude of a star is the apparent magnitude that it would have IF it was observed from a distance of 10 parsecs- most stars are further than that so they would be brighter if observed from a distance of 10 parsecs. This means their absolute magnitudes are more negative than their apparent magnitudes.
Good, concise description. Can you post a link to the applet you used to generate the spectrum in E2.7 ?
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