# Can someone explain relative brightness and parallax to me? (OCR 21st Century)

Announcements
#1
I've got a parallax diagram in front of me so don't worry about that...To be honest I understand parallax a bit but I just can't get my head round relative brightness. It's so confusing.
0
7 years ago
#2
(Original post by icy_typhoon)
I've got a parallax diagram in front of me so don't worry about that...To be honest I understand parallax a bit but I just can't get my head round relative brightness. It's so confusing.
Are we talking about the physics terms?
0
#3
(Original post by Andy98)
Are we talking about the physics terms?
yeah, the two methods of measuring the distance to stars.
0
7 years ago
#4
(Original post by icy_typhoon)
yeah, the two methods of measuring the distance to stars.
Right... Well my understanding isn't top notch but Ill give it a go:

Parallax is just the apparent movement of a near object against a further away one - like if you stick your thumb up and close one eye at a time (the thumb looks like it's moving). The opposite sides of Earth's orbit (winter solstice and summer solstice) are like switching eyes - the closer stars appear to move in relation to the ones that further away. The true position of the star (relative to our sun) is slap-bang in the middle of the two extremes. See: (http://hyperphysics.phy-astr.gsu.edu...st/stelpar.gif) this diagram. To find distance of said star from the sun use the formula included.

As for apparent brightness:

Spoiler:
Show
The apparent magnitude (m) of a star is a measure of its apparent brightness as seen by an observer on Earth. The brighter the object appears, the lower the numerical value of its magnitude. Absolute magnitude is a measure of luminosity, how much light a star radiates into space. Absolute magnitude can be defined as apparent magnitude a star would have 10 pc away from Earth. A star that looks just as bright as one close to us, but being further away has a greater absolute magnitude. The apparent magnitude represents the apparent brightness and the absolute magnitude the absolute luminosity. The absolute magnitude of a star is the apparent magnitude that it would have if it were observed from a distance of 10 parsecs.
The magnitude scale has been used by astronomers for more than 2000 years to classify stars into 6 categories of brightness as it appears to the human naked eye, with magnitude 1 being the brightest and magnitude 6 being the faintest. With the use of telescope and more sensitive instruments, stars beyond magnitude 6 can now be more accurately measured.
The magnitude scale is now defined as magnitude 1 being 100 times brighter than magnitude 6, with the scale being logarithmic. Negative values for stars brighter than magnitude 1 is also allowed. With the difference of magnitude 1 and 6 being defined a 100, this means the each unit decrease of the magnitude scales corresponds to 2.512 times brighter (as 2.5125=100).
For example, to compare the power received from the two stars Sirius, with apparent magnitude -1.46, and Betelgeuse, with apparent magnitude 0.5, raise 2.512 to the difference of the apparent magnitude.

Difference in apparent magnitude of the two stars = 0.5 – (-1.46) = 1.96

Therefore power received from Sirius/power received from Betelgeuse = 2.512 x 1.96

While the apparent magnitude described above is the brightness apparent to an observer on Earth, absolute Magnitude is defined as the apparent magnitude of a star at a fixed distance of 10 parsec. Since the apparent brightness of a star depends on the absolute brightness and distance of the star, the relationship is given by the following formula known as the distance modulus.

where M is the absolute magnitude
and m is the apparent magnitude
and d is distance measured in parsec

If the luminosity of a star is known, the inverse square law relating apparent brightness, luminosity and distance can be applied to estimate the distance to the star. The formula being:

b is apparent brightness (m)
L is luminosity (W m-2)
d is distance (m)
r is radius of star (m)

No clue if that helps.

It looks like apparent brightness is....well, exactly that. They look at a star and say "It looks brighter than that one so it must be closer".

There is another one which is similar to this called absolute brightness, which is basically where they figure out how much light the star actually emits - but I have no clue how that one works.

Are you sure this is GCSE stuff? We never did anything like this.
0
#5
(Original post by Andy98)
Right... Well my understanding isn't top notch but Ill give it a go:

Parallax is just the apparent movement of a near object against a further away one - like if you stick your thumb up and close one eye at a time (the thumb looks like it's moving). The opposite sides of Earth's orbit (winter solstice and summer solstice) are like switching eyes - the closer stars appear to move in relation to the ones that further away. The true position of the star (relative to our sun) is slap-bang in the middle of the two extremes. See: (http://hyperphysics.phy-astr.gsu.edu...st/stelpar.gif) this diagram. To find distance of said star from the sun use the formula included.

As for apparent brightness:

No clue if that helps.

It looks like apparent brightness is....well, exactly that. They look at a star and say "It looks brighter than that one so it must be closer".

There is another one which is similar to this called absolute brightness, which is basically where they figure out how much light the star actually emits - but I have no clue how that one works.

Are you sure this is GCSE stuff? We never did anything like this.
Thanks! That helps a LOT. (Didn't need the spoiler bit though lol, that looks like university level to me)
and yeah, P1 Earth in the Universe OCR 21st Century. Part of the module is about measuring the distance to stars. -_- I hate physics...
0
7 years ago
#6
(Original post by icy_typhoon)
Thanks! That helps a LOT. (Didn't need the spoiler bit though lol, that looks like university level to me)
and yeah, P1 Earth in the Universe OCR 21st Century. Part of the module is about measuring the distance to stars. -_- I hate physics...
Wow...I did edexcel and the most complex thing we had to do with stars was..."What is redshift?" And yeah I love physics
0
#7
(Original post by Andy98)
Wow...I did edexcel and the most complex thing we had to do with stars was..."What is redshift?" And yeah I love physics
red shift is eaaaasy
ha lucky! It's my worst subject XD
0
7 years ago
#8
(Original post by icy_typhoon)
red shift is eaaaasy
ha lucky! It's my worst subject XD
You're right there.

It's my best subject
0
X

new posts
Back
to top
Latest

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### Poll

Join the discussion

Talking to current university students (0)
0%
Talking to peers going through the same thing (0)
0%
Speaking to student ambassadors from the universities (0)
0%
Speaking to staff members from universities (0)
0%
Using the personal statement builder, library or helper service (0)
0%
100%
Learning about/speaking to Student Finance England (0)
0%
Something else (tell us in the thread) (0)
0%