Chapter 9.2: Stars Flashcards
What is absolute and apparent magnitude?
Apparent magnitude (m): how bright the star appears from Earth.
Absolute magnitude (M): how bright the star would appear if it were placed 10 parsecs from Earth.
What is the Hipparcos scale?
Brightest stars have a apparent magnitude of 1 and faintest, 6. Has now been extended to include the sun (-26.47) and fainter objects discovered with the telescope.
Define parsec.
The distance to an object that subtends an angle if 1 arc second (1/3600 of a degree) to the line that runs from the centre of the Earth to the centre of the sun.
Define luminosity in the context of stars.
Luminosity is the rate of light energy released or the power output of a star.
Explain the concept of intensity as it related to stars.
Intensity is the power received from a star (it’s luminosity) per unit area, measured in watts per square metre (W m^-2)
How does the inverse square law apply to the intensity of stars?
The intensity of a star is inversely proportional to the square of the distance from the star.
Define light year.
The distance travelled by light in a vacuum in one year. (Speed of light x no. seconds in 1 year)
State Stefan’s Law:
The power output of a star is directly proportional to its surface area and its (absolute temperature)^4.
State Wien’s Law.
The wavelength of a star’s emission at peak intensity is inversely proportional to its absolute temperature.
What is a black body?
A black body absorbs EM radiation of all wavelengths and can emit EM radiation of all wavelengths. A black body does not reflect any radiation — it absorbs all radiation incident on it.
What is the order of spectral classes?
O(oh)
B(balls)
A(a)
F(fucking)
G(giant)
K(killed)
M(me)
Draw a table to show the characteristic of each stellar spectral class, including colour, temperature and absorption lines.
Find an image pls.
How are each of the spectral classes related to Balmer lines?
O - weak: star’s atmosphere too hot hydrogen likely to be ionised
B - slightly stronger: “”
A - strongest: high abundance of hydrogen in n=2 state
F - weak: too cool, hydrogen unlikely to be excited
G, K, M - very weak/ none: too little atomic hydrogen, far too cool to be excited
Draw the hertzsprung-russell diagram.
Look it up
Annotate the Hertzsprung-Russell diagram to show the Sun’s evolution.
Look it up
What are supernovae? Describe how type Ia and type II form.
A supernova is the explosion of a star, which causes is to very suddenly and rapidly increase in absolute magnitude.
Type Ia: The result of a white dwarf core accumulating too much matter from it’s binary partner and exploding above a critical mass.
Type II: A single star (eg a red giant that collapses rapidly under it’s own gravity, causing its outer layers to be ejected.
Explain why type II supernovae cannot be used as standard candles whereas type Ia can.
A standard candle is an astronomical object that has a known absolute magnitude so astronomers can calculate the distance using m-M = 5log(d/10). All type Ia supernovae explosions have the same peak absolute magnitude (approx. -19.3) as they all have the same critical mass (thus consistent light curves) so they can be used as standard candles. Type II supernovae are not as predictable so they cannot be used as standard candles.
Draw the light curve of a typical type Ia supernovae.
Look it up
What is a black hole?
When the core of a star is larger than 3 solar masses collapses, it forms a black hole. The escape velocity of a black hole is greater than the speed of light so light cannot escape it.
The boundary at which the escape velocity equals the speed of light is called the event horizon, and the distance from the centre of the black hole to the event horizon is called the Schwarzchild Radius.
What is dark energy?
- when astronomers calculated the distance to some type Ia supernovae, they discovered them to be dimmer than expected. This suggested the expansion of the universe is accelerating, which has been attributed to dark energy.
- dark energy is though to be energy that has an overall repulsive effect on the universe.
How do you calculate the radius of a black hole.
Schwarzschild radius.
