gravity and distances

Question 1

Galaxies defenition

Answer 1

• structures of stars and clouds
• bound by their mutual gravity

Question 2

What are brown dwarfs?

Answer 2

• more massive than planets but not massive enough to be stars
• can have nuclear fusion involving deuterium but not hydrogen
• there isn’t much deuterium in space so it quickly runs out

Question 3

why is the unit of a lightyear hardly used in astrophysics

Answer 3

the length of a year is not strictly defined

Question 4

what is the defenition of one astronomical unit (AU)?

Answer 4

• the distance between the sun and the earth
• the distance changes as the orbit of the earth is slightly elliptical
• the AU is the average distance

Question 5

number density defenition

Answer 5

the number of particles (or any countable objects) per unit volume

Question 6

planets and related objects (moon, comets)

Answer 6

small, solid or gaseous and do not support nuclear fusion

Question 7

stars

Answer 7

• large and gaseous
• generate their own energy through nuclear fusion
• a black hole formed from a collapsed star is still considered a star

Question 8

nebulae

Answer 8

• clouds of gas (mainly hydrogen) and some dust
• these clouds can collapse to form new stars

Question 9

what is the weakest of the fundamental forces?

Answer 9

gravitational force

Question 10

equation for gravitational force

Answer 10

GmM/r^2

Question 11

formula for gravitational acceleration

Answer 11

GM/r^2

Question 12

equation for the work done by gravity

Answer 12

W = -GMm(1/r2 - 1/r1)

Question 13

What is the potential energy of an object in a gravitational field, include the equation?

Answer 13

EP = -W
EP = -GmM/r
- energy required to move a mass of one kg from a point in a gravitational field to infinity

Question 14

what is the escape velocity? no need to memorise equation, just derive it yourself.

Answer 14

• the velocity needed for an object to reach infinity in a gravitational field, it is irrespective of the mass of the object
• derive by equating PE to KE

Question 15

What is the orbital velocity? defenition. dont memorise equation, just derive it.

Answer 15

• the velocity needed to be in a stable orbit around a mass M at a radius r from the centre
• derive by equating gravitational and centripetal force

Question 16

how did the greeks measure the circumfrence of the earth?

Answer 16

• found distance between two cities
• measure the angle the suns rays makes from the zenith at one city when the sun is directly overhead the other city
• this angle is equal to the angle between the two cities from the centre of the earth, easy calculation from here

Question 17

how did the greeks measure the distance to the moon

Answer 17

• during eclipse, the moon travels through the shadow of the earth
• since the sun is much further than the moon, it can be assumed that the shadow cast is cylindrical
• calculate time in shadow, u know the period of orbit (29 days)
• use s = d/t and solve for d to find the circumfrence of orbit. solve for the radius.

Question 18

how to find distances using parallax? dont memorise equation, derive!!

Answer 18

• measure the change in angle when taking a small sidestep.
  use trigonometry, let b be the baseline, d be the distance and theta be the change in angle

Question 19

what is the maximum value of b when measuring distances using parallax on the earth

Answer 19

• the diameter of the earth, obviously
• however, the earth is moving so wait until it moves to the other side of the sun. then the largest baseline possible is 2AU which allows you to measure very large distances

Question 20

what is the simple equation for the distance using parallax

Answer 20

d (parsecs) = 1/ p (arcseconds)

Question 21

what is the distance when the parallax is one arcsec

Answer 21

one parsec

Question 22

how many degrees is one arcsecond, and one arcminute?

Answer 22

• arcsecond = 1/3600 degrees
• arcminute = 1/60 degrees

think about it like how theres 3600 seconds in an hour, and 60 minutes in an hour

Question 23

what is the defenition of the angular extent? equation of angular extent.

Answer 23

• angular extent/ angular size : measure of how large an object appears in the sky, as seen from a particular observation point. tells you the angle the object subtends, gives idea of height
• ratio of linear size (r) and distance (d) from the observer
• theta (RADIANS) = r/d
• theta has to be small as small angle approx is used, theta must be less than one radian otherwise use tan(theta) instead.

Question 24

what is the declination (dec) when using coordinates to locate something in the sky?

Answer 24

• like earths latitude, tells you how far north or south something is in the sky
• +90 degrees is the north celestial pole, directly above the earths north pole
• -90 degrees is the south celestial pole, directly below the earths south pole
• declination values range from -90 degrees to +90 degrees

Question 25

what is the right ascension (RA) when using coordinates to locate something in the sky?

Answer 25

- instead of dividing the sky into 360 degrees, astronomers divide into 24 hours like a clock

Question 26

what is the relationship between the change in angle in seconds, and the change in angle in arcsecs in the coordinate system

Answer 26

delta alpha (arcsecs) = 15 (delta alpha (seconds)) (cos delta) where delta is the declination

Question 27

what is proper motion and what is the unit that it is measured in ?

Answer 27

stars have velocities relative to eachother and we see them change position in the sky. this is measured in arcsecs per year

Question 28

what is the defenition of the luminosity of a star

Answer 28

the amount of energy that the star emits per second, it is a power and has the units of watts

Question 29

what is the defenition of the flux

Answer 29

F - the energy per second per square metre received from the star at the detector, units watts per metre square

Question 30

what is the magnitude of a star and how is it related to the flux?