Distances and Magnitude Flashcards
What’s it the luminosity of a star?
The total energy it emits in the form of electromagnetic radiation per second
Measured in watts
It’s the power output of a star
What is the intensity of an object that we observe?
Power received from it per unit area at earth
What is the effective brightness of an object?
Its intensity
What is apparent magnitude?
How bright the object appears from earth
What does the brightness of a star in the night sky depend on? (ignoring weather and light pollution)
Its luminosity and its distance (closer = brighter)
What is the scale of apparent magnitude?
1 = brightest 6 = dimest
A magnitude 1 star has an intensity how many times greater than a magnitude 6 star?
100 times
What’s the equation for calculating brightness ratio between 2 stars?
I2 / I1 = 2.51^m1-m2
Difference of 1 on the apparent magnitude scale is 100^1/5 = 2.51
Brightness of sun?
m = -26.7
Dimmest objects Hubble telescope can see on scale?
m = 30
What planet, with m=0, is the apparent magnitude scale tied to?
Vega
What is a parallax?
The apparent change in position of an object as the observer moves position.
Parallax is measured in?
An angle
How is the distance to nearby stars calculated using parallax?
What is the distance between each end of earths orbit
6 months
What is the angle of parallax?
When you view a star from each end of earths orbit (6 months apart), the angle in-between the earths positions
(6 months apart) and the star, divided by 2 is the angle of parallax. Its also the angle between the sun and the earth when the angle is at the star.
What happens to parallax angle when the star is closer
Bigger angle of parallax
If you know the angle of parallax what do you do to find the distance between the earth and the star?
Trigonometry:
How do you find the distance between the earth and star if the angle is small?
Presume tan(x) = x
1 arc second = X degrees?
X = 1/3600
1 degree = X arcseconds?
X = 3600
How many parsecs away is a star if it’s parallax angle is 1 arcsecond
1 parsec
Absolute magnitude is and isn’t based on what?
Only luminosity (or power output), not the distance.
What is absolute magnitude defined as?
what its apparent magnitude would be if it was 10 parsecs away
What is M, m and d in the brightness equation
Why can’t parallax be used to measure the distance to most stars?
They are too far away
What is a standard candle? Whats an example?
An object that you can calculate the luminosity directly e.g. type 1a supernova
All type 1a supernovae have the same what?
Same peak in absolute magnitude = -19.3
What is the rearranged brightness equation?
1 AU is defined as what?
The mean distance from the sun to the earth
What is a light year?
The distance that electromagnetic waves can travel in one year, in a vacuum
If we see light from a star 10 light years away, we are seeing it as if it were X years ago/ahead
Seeing it as if it were 10 years ago.
Further away an object is further back in time we see it.
How do you work out half the angle subtended by the object at the eye, knowing the distance away and radius of the object?
When is parallax used?
When the object is nearby
When is the angle subtended used? Why is it unlikely that you know it?
If we know the objects size. It’s unlikely we have the size if we don’t know the distance
Why is using other methods like red-shift, standard candles and quasars useful for measuring distance?
Several ways can make it more precise
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