7.3 - Mapping the Universe Flashcards
Briefly, what is parallax?
An apparent change in position of an object against a distant background.
In astronomy, what is a parallax angle?
HALF the angle moved against distant background stars over 6 MONTHS. The NEARER an object is to you, the GREATER the angle.
What are parallax angles measured in?
Arcseconds (“)
What is one Arcsecond equal to?
1/60 of a minute = 1/60 of a degree
What is a parsec (pc) ?
The distance to a star with a parallax angle of 1 arcsecond.
What is 1 parsec equal to in LIGHT YEARS?
3 ly
What is the formula for calculating a distance in parsecs?
Distance (pc) = 1 / parallax angle (ARCSECONDS)
Why is calculating Parallax not really effective for ALL stars?
It can only be used to identify nearby stars. As the more distant the star is, the less it appears to move so it is less noticeable to measure.
What is an Interstellar Distance? How many parsecs is it?
The distance between stars = A few parsecs
What is an Intergalactic Distance? How many parsecs is it?
The distance between galaxies = A few MEGAparsecs
How many Megaparsecs (Mpc) are there in parsecs?
1 000 000 (1 million) Parsecs.
What is “luminosity”?
The total power emitted by a star.
What does the luminosity of a star depend on?
- Its size (larger star = greater surface area to emit radiation)
- Its temperature (If temperature increases = radiation emitted increases)
What is “observed intensity / brightness” ?
How much energy emitted by a star REACHES THE EARTH.
What law does the spreading of radiation from a point source obey?
The INVERSE SQUARE law
Doubling distance = Spreads out 4x the area = 1/4
Tripling Distance = Spreads out 9x the area = 1/9
What is a ‘period’ of a star?
The time taken to complete one full cycle.
What is a Cepheid Variable star?
A star where its luminosity varies in a regular pattern, brightening and dimming again every few days.
How do astronomers find the distance of a really distant star?
- Use parallax to calculate the distance to nearby Cepheid Variable stars.
- Use that information and how the bright the star appears to be to calculate the luminosity.
- Link this to the period of the star to calculate how the luminosity depends on the period.
- Estimate the distance to Cepheid Variable stars that are too far away for the Parallax method to be used.
- By studying the star to find its period you can work out its luminosity.
If we know the luminosity of a star, what information can that give us?
We know how much light it gives out. By comparing this to how much light actually reaches Earth we can calculate how far away the star is.
When looking at Cepheid Variable stars of the same period, the ________ the star, the ______ away it is.
The fainter the star the further away it is.
This is because they have the same period so they must have the same luminosity.
What allowed an estimate of the size of a galaxy to be determined?
Cepheid Variable stars.
What did Harlow Shapley use Cepheid Variable stars (in groups of stars called globular clusters) ?
To find their luminosity and the distance to them.
What did Shapley assume when using Cepheid Variables?
That all globular clusters were the same luminosity and used this to measure the distance to many different clusters.
The use of telescopes led to the discovery of fuzzy bright clouds of light. What are these scientifically called?
Nebulae
