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Learning objectives

By the end of this section, you will be able to:

  • Explain the difference between luminosity and apparent brightness
  • Understand how astronomers specify brightness with magnitudes

Luminosity

Perhaps the most important characteristic of a star is its luminosity    —the total amount of energy at all wavelengths that it emits per second. Earlier, we saw that the Sun puts out a tremendous amount of energy every second. (And there are stars far more luminous than the Sun out there.) To make the comparison among stars easy, astronomers express the luminosity of other stars in terms of the Sun’s luminosity. For example, the luminosity of Sirius is about 25 times that of the Sun. We use the symbol L Sun to denote the Sun’s luminosity; hence, that of Sirius can be written as 25 L Sun . In a later chapter, we will see that if we can measure how much energy a star emits and we also know its mass, then we can calculate how long it can continue to shine before it exhausts its nuclear energy and begins to die.

Apparent brightness

Astronomers are careful to distinguish between the luminosity of the star (the total energy output) and the amount of energy that happens to reach our eyes or a telescope on Earth. Stars are democratic in how they produce radiation; they emit the same amount of energy in every direction in space. Consequently, only a minuscule fraction of the energy given off by a star actually reaches an observer on Earth. We call the amount of a star’s energy that reaches a given area (say, one square meter) each second here on Earth its apparent brightness    . If you look at the night sky, you see a wide range of apparent brightnesses among the stars. Most stars, in fact, are so dim that you need a telescope to detect them.

If all stars were the same luminosity—if they were like standard bulbs with the same light output—we could use the difference in their apparent brightnesses to tell us something we very much want to know: how far away they are. Imagine you are in a big concert hall or ballroom that is dark except for a few dozen 25-watt bulbs placed in fixtures around the walls. Since they are all 25-watt bulbs, their luminosity (energy output) is the same. But from where you are standing in one corner, they do not have the same apparent brightness. Those close to you appear brighter (more of their light reaches your eye), whereas those far away appear dimmer (their light has spread out more before reaching you). In this way, you can tell which bulbs are closest to you. In the same way, if all the stars had the same luminosity, we could immediately infer that the brightest-appearing stars were close by and the dimmest-appearing ones were far away.

To pin down this idea more precisely, recall from the Radiation and Spectra chapter that we know exactly how light fades with increasing distance. The energy we receive is inversely proportional to the square of the distance. If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. If it is three times farther away, it will look nine (three squared) times dimmer, and so forth.

Questions & Answers

something's like that maybe, the ways to know them
Dennis Reply
oh ok you can actually reply to your own comment by tapping it once
Tanmay
you should do this
Tanmay
what are laws of the universe
Dennis Reply
The law of attraction
Lambent
Are you talking about fundamental forces Dennis?
Tanmay
you can go to google and type 4 fundamental forces of the universe
Tanmay
let me give you gist of it there are 4 fundamental forces namely gravitational, electromagnetic, strong nuclear and weak nuclear
Tanmay
the came into being just after the bid bang
Tanmay
I mean big bang
Tanmay
you should watch this video for better understanding ***youtu.be/X9otDixAtFw
Tanmay
wow some more than information
Rajeshwari Reply
since space gets bigger dose it affect distance between the sun and earth
Swazi Reply
yes the distance between earth and sun increase at a rate of 15 cm per year
Tanmay
ohh really I didn't knew that , you learn something new everyday
NARUTO
true
Tanmay
thanks so much my friend
Youssef
Dear tanmay and other friends I want to ask that if both sun and earth are moving away from their center than why their inter relative distance is increasing rather squeezing taking in consideration of masses I am student so I apologize further and wanna guidance if possible
haseeb
The Earth does not get close to or “touch” the Sun because it is in orbit around the Sun. A circular orbit means that the orbiting body always stays the same distance from the thing it orbits. The Earth's orbit is nearly circular and keeps us about 93 million miles from it.
Konica
Konica nor the orbit can prevent our earth from getting closer (if it was coming closer) and nor is the orbit of earth nearly circular
Tanmay
And Haseeb can you make your question a little simple or something I didn't get it. What do you mean by moving away from their center?
Tanmay
And then comes your increasing rather than squeezing taking mass into consideration I never heard of that and I didn't understand that
Tanmay
Which formula are you using for that squeezing part?
Tanmay
What's difference between Astronomy and Astrophysics?
Gurjinder Reply
Actually astrophysics is one of the many branches of astronomy. It usually deals with the mathematical calculations of predicting the motions of heavenly bodies and also it studies the evolution of our universe by using physics.
Debabrata
So is there any observational astrophysics?
Gurjinder
Yes there are 2 types of astrophysicsist one is theoretical and other one is observational. I don't know about astrophysics though
Tanmay
Thanks
Gurjinder
welcome
Tanmay
😊
Gurjinder
Is there a way for us to find out that we are in a simulation or not?
Tanmay Reply
I think mathematics is a key
Arghyabindu
If we are in a simulation then I don't think that the super intelligent species which is controlling this simulation will give us any hint about how to know it.
Debabrata
There has to be some kind of bugs even a carefully designed app has some bugs in our world
Tanmay
Yeah.... There might be some loophole in their programme code.... And who knows.... We could find the bugs in a super massive black hole
Debabrata
But here we are talking about a very very very massive simulation by a very advanced species...... So they might ve already planned on how to deal with all the bugs and glitches
Debabrata
There's a really interesting theory that states even if we are in a simulation, it would be coded in such a way that as soon as one of us (a sentient being inside the simulation) had the slightest hint/evidence that we were in a simulation, the simulation would be edited to remove that information
ben
(I keep commenting on posts and they just dont post sometimes, does anyone else have this problem or should I just delete the app?)
ben
same happened to me but I see yours))
NARUTO
(ah well it must just be a little problem with the app, cheers for letting me know 🙌)
ben
(Well never happened to me though it takes some time to post large messages)
Tanmay
I saw a comment where he told that the universe is a game and finding it's nature is the level as soon as we find its nature the universe collapse and make another universe with more complex nature and this continues
Tanmay
(ahh that may be what it is actually because it is only the longer messages, thanks for letting us know)
ben
That was really a unique way of seeing the universe
Tanmay
Impractical way are there such as leaving your mortal body and entering to the astral body....cons of this idea is you can't tell us what you have saw or feel in that endemic confrontation
Arghyabindu
what a friend told me now
Arghyabindu
We are just a component of the universe trying to understand the other components of the universe...... So I guess that sometimes it makes the matter more and more complicated
Debabrata
what a friend told me now
Arghyabindu
Wait if we are in a simulation then does that means our future is predicted and we can't change it like some kind of rpg game
Tanmay
Yes IF we are in a simulation then definitely the future is already programmed..... But it may be in early access stage..... And the future might not be written at the moment.
Debabrata
so we can do time travel upto the point where our future is written
Tanmay
Maybe or may not be..... Depending on the script or program written in the simulation..... The intelligent species should be controlling every step we take and every thought that come into our mind.
Debabrata
Have you seen sword art online anime?
Tanmay
well never mind it's just repeated stuff
Tanmay
Nah.... I don't watch anime
Debabrata
ok
Tanmay
hii guyz
Lambent
Hey
Debabrata
BLM ✊🏾✊🏾✊🏾
Laila
?
Kilten
which is the biggest star in the universe? if anyone know answer,please reply.
Ciffee Reply
which is the biggest star. ?
Ciffee
u y scuti
Sunidhi
sun
Konica
for our galaxy
Konica
Konica UY Scuti is the biggest Star ever observed and it's located in our galaxy.
Gurjinder
what are the branches of astronomy?
SCIENCE Reply
So there are literary many branches of astronomy like- Radio astronomy, Ultraviolet astronomy, X ray astronomy, etc. (just imagine the electromagnetic spectrum) There's also Astrophysics (My favorite), astro chemistry, astrobiology, astro geology, planetary science, astrometry
Debabrata
factors that contribute to the development of ancient astronomy to modern astronomy
Andrea Reply
When will be we found them?I am so waiting.
Everything Changes when Galileo Galilee observed Sky with a telescope. There's no doubt that ancient astronomers knows a lot of things about sky , but there observation was with naked eye. i wanna share some of the glimpse of Indian ancient astronomy:
Gurjinder
1.They know the difference between the Star and Planets at that time. 2. They know which star is multiple star system and their rotation with respect to each other. 3. They know the value of speed of light. And their are lots of things which they knows even without telescope.
Gurjinder
why is a supernova
pooja Reply
well there are different types of supernovas. there is a type 1a supernova and that happens when a white dwarf reaches the mass of 1.4 of the sun. A type 2 supernova is when a high mass star starts to fuse iron in the core and the star collapse then expands to create that supernova
PopularHellboy
what is time.!
Samyak Reply
92 billion light years
The
4th Dimension
The
time is illusion
Ciffee
Time is a way of describing an event happened in our 3D world
Tanmay
Why are the largest visible-light telescopes in the world made with mirrors rather than lenses?
Nibha Reply
What is a charge-coupled device (CCD), and how is it used in astronomy?
Nibha
Why is it difficult to observe at infrared wavelengths? What do astronomers do to address this difficulty?
Nibha
not sure what a CCD.
PopularHellboy
(also, lenses have are more problematic over larger distances due to the way different wavelengths of light act within glass. Red and blue light wavelength differs by about 300nm, and so the refraction will affect them differently, whereas when reflecting on a mirror, this is not a problem)
ben
hi
Zalka
i'm from Indonesian
Zalka
(I tried posting this before but this app doesn't seem to work properly) A CCD essentially works as a camera. It's made from an array of capacitors (which make up the 'pixels'). When an incident photon hits the 'pixel', the charge generated is measured and can be translated into an image we can view
ben
the reason mirrors are more common than lenses (on large scale telescopes) is because it is a lot easier and cheaper to make a parabolic mirror than it is to make a perfect glass lens
ben
Because Refracting Telescope cause Cromatic abberetion.
Tushar
Just reading along. I can't see the end of the page so commenting to move it up.
Eva
same. but mirrors are also made with great efficiency and calculation for trapping the light
Konica
How might Venus’ atmosphere have evolved to its present state through a runaway greenhouse effect?
Nibha Reply
what is synchrotron radiation
Nibha
I am 15 and I got interested in astronomy. Can anyone tell me the basics that I must know?
Vikky Reply
you can start by studying the theories of astronomy. there is light and the spectrum of electromagnetic radiation. from there you learn about waves and how we as astronomers received information from space. then you can learn about the nebula hypothesis and also about our solar system
PopularHellboy
but if you learn about the foundation of light and the dually of nature with it and the Doppler effect, it will apply to most stuff in astronomy
PopularHellboy
heyy @populadHellboy can you tell me from where can I find this Light and The spectrum of electromagnetic radiation ? I mean is there any course ?
Sagar
I mean I learn astronomy in college with astronomy 101 and 102 I'm sure if you search online for YouTube videos or blogs they can help you with that. try crash course on YouTube for an introduction
PopularHellboy
Currently preparing for olympiad, is there any online resources for Astronomy, astrophysics, and cosmology? If there's any past question papers i'll be delighted. I'm currently using Principle and Practice and Introduction To Modern Astrophysics..
Aditya
what is time
Sourav
how big is a parsec
David Reply
I believe it's about 3.086 x 10^16 meters. You can try converting 1 radian to 1 arc secs, multiply with 1 AU, convert it to meter.
Aditya
Its 31 trillion km
Asim
very big
Ciffee
Practice Key Terms 3

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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