The Universe - Light Years + Brightness Flashcards
What are Ultra Deep Field images, and how are they taken?
The Hubble telescope has captured Ultra Deep Field images where each dot is a galaxy. Some are as far as 13b light years away from Earth. It was taken from a tiny empty patch in the sky over one million seconds.
What is a light year?
A light year is distance travelled by light in one year.
How many light minutes away from us is the Sun?
8.4
Why are distances in light years logarithmic?
To allow for large variation in distances.
What are Hertzprung-Russell diagrams?
Hertzprung-Russell Diagrams compare stellar luminosity or logluminosity, (y axis), to surface temperature or logtemp in K.
Describe Rigel’s fuel burning and compare it to the Sun in brightness.
Rigel, the blue giant, is the hottest and largest star. It burns through fuel quickly and finally becomes a supernovae where it briefly outshines galaxies during energy release. It’s 66,000* brighter than the Sun.
What is radar and when was it first used?
Distance is measured via RADAR, (radio detection and ranging), which measures distances in the same way air traffic controllers and traffic police do. In 1961 radar was first used to send and receive a signal from Venus.
What are limits to radar?
- Signals are weak and cannot be sent long distances, (range relies on a powerful transmitter and large antenna - power-aperture output).
- Noise from electrons at the receiver interferes with signals but matched filters for signals can stop this.
- Echoes from internal and external sources can limit radar capability, but Doppler frequency shift can distinguish moving signals from those stationary.
How can distance be found to an object?
To find distance to an object, use s=o.5ct.
What was the first estimate for distance to the moon and how was the modern value obtained?
Hipparchus, an astronomer, first estimated the distance to the moon in the 2nd century BC, 410^8m. The modern value is 3.8410^8m. This was obtained from lidar - light detection and ranging.
How is lidar done?
- Emit laser light from Earth station, this hits reflectors on lunar surface left by Appollo astronauts
- Receiver gets return signal which is weak, but laser properties can be identified.
How can you find relative velocity of a moving object?
- Send pulse, and time its journey to find s1.
- Wait a time interval of T seconds.
- Send second pulse and find s2.
- Use v=s1-s2/T to find relative velocity of a moving object.
What assumptions are made?
Assume speed of signal is same both ways, reflection occurs halfway through pulse’s journey, and speed of light is constant no matter the perspective, (relativity).
What’s the formula for flux density/luminosity/brightness?
P/4A (P=power, A=area). Use P/A for an object where you have its area and neither object is moving.
