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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

I l don't understand questions
Nkateko Reply
which quesrions?
Ankit
How to make a telescope
Amit Reply
***youtube.com/channel/UClBN_RpR5X8IdrARwnaL3fg
Steve
***wikihow.com/Make-a-Telescope
Jennifer
optical mechanics
Astro
aptical
Nkateko
Which one has the ring
Isaac Reply
Saturn venue and even Jupiter all have rings the rings on Jupiter, however, aren't so easily noticed due to them only being viable through infrared
Hunter
visible *
Hunter
they each have three rings but not as you might think just think of them all as one big ring because that's basically what they are but they are desperate individual rings
Hunter
separate*
Hunter
can you tell me what is a quantam elighment? Lets see how many people give write answers
gdjm Reply
can you tell me what is a fomalhaut?
gdjm
Fomalhaut Is The Brightest Star In The Constalellation Of Piscis Austrinus.....
Adam
joe.
Tracy
Do You Mean Quantum Entanglement?
Adam
yes
gdjm
No Adam fomalhaut mean eye of universe
gdjm
quantum entanglement is when a husband has been married so long to his wife that he is directly influenced by her action usually in an opposite way. 🤣
Jocofranc
It Is Also The Brightest Star Of The The Constalellation Of Piscis Austrinus.....
Adam
who was the first person to get to the planet
Isaac
can anyone tell me daily rotation of the sun in tokyo
Liridona Reply
tokyo has it's own personal sun
Tracy
can you send me any pictures of the tokyo sun
Liridona
sure! ig?
Tracy
here can u?
Liridona
no it wont let me sadly
Tracy
or
Liridona
snapchat
Liridona
i dont have snapchat 😰
Tracy
okay ig
Liridona
my ig is fatwigga37 if you'd like to see :)
Tracy
Can we use SI units in the equation of Kepler's third law?
Naiyu Reply
can u tell the equation pls
sasmita
how does religion enters to this topic when god created this world
Sakira
How Do You Know A God Created Our Planet?
Adam
you must read bible
Sakira
u must the bible
Sakira
How Do You That Book Is Authentic? And I Have Read The Bible, The Koran, The Torah And The Urantia.....
Adam
The law is T^2 is proportional to R^3 where t is planet's time period and r is average of semi major axis and semi minor axis. Also,don't turn this place into a religious warzone. We already have idiots fighting about religion.
priyanshu
If I Had A Book That Said Insects Created The Universe, Would That Make It Fact?
Adam
Have You Ever Heard Of The Council Of Nicaea?
Adam
Through Out History Mankind Has Worshipped Over 5000 Different Gods, How Do You Know Christ Is The True God?
Adam
God Is A Hypothesis..... The Definition Of Faith Is Belief Without Proof.....
Adam
Thank You Prianshu Roy, This Is A Forum For Science Not Archaic Stories Written By Sheep Herders, Well Said.....
Adam
Hebrew 13:1
onome
P2 = a3
Adam
So You Quote A Bible Verse, Now What?
Adam
Science Discourse Nothing Else.....
Adam
god is real he's not just a wise guess
Sakira
god bless you
Sakira
u know what,u are not here if god werent there!
Sakira
Science Discourse, Nothing Else.....
Adam
Thier Are Many Religious Sites You Can Go To, To Discuss God.....
Adam
hahahaha
Sakira
Have A Great Day.....
Adam
night time here
Sakira
Look Up The Definition Of Day, Instead Of Trying To Look Witty, Again Have A Great Day.....
Adam
😈😈😈
Sakira
such an socratic person?
Sakira
What Was Your Intention In Stating "Night Time Here" ?
Adam
please don't fight over silly stuffs!
Romancho
tangina mo
Sakira
No I Don't Believe A God Exist, But I Will Fight For Your Right To Worship Any God You Wish.....
Adam
no fighting
Sakira
But This Venue Is For Science Discourse.....
Adam
i know!
Sakira
No Fighting, Just Discussion.....
Adam
yes Naiyu, we have to use SI unit in the equation for the Kepler's every laws
Madhav
Thanks.
Naiyu
wow
Kenneth
what's the scene of voyager 1,did we get any information through it
Aarushi Reply
we are getting a lot of info yet more to be received check official mission report from the officials site ....
Ganeshan
what are the best astronomy books?
neehar Reply
what is the parallel space?
neehar
how to measure the brightness of a star in w\m^2
Aarushi Reply
it's based on the percentage of helium I think
neehar
ok TQ😊
Aarushi
it's based planet colour and brightness which called lumiosity
gdjm
Are there planets that has the same characteristics as earth.
Nito Reply
yes ofcourse there are earth like planets but they aren't completely like earth.still scientists have a hope of life existence in there.
Aarushi
yeah of course example is Mars but they only discover water at mars
neehar
there's a much possibility of presence of earth like planets in the universe if they were present at goldilock zones
Jashwanth
what are goldilock zones?
Aarushi
why yes there are planets just like earth out there somewhere we know that much for sure given the number of planets in the galaxy, as a matter of fact, one of Jupiter moons officially named "gamamede" is very much like earth in size and characteristics
Hunter
How to become an astronomy
Aakarsh Reply
astronomer*
Emilya
Yes
Aakarsh
Astronomer
Aakarsh
yes i Also want to know the answer
Mehar
How to become?
Aakarsh
Me too
Romancho
By hard study
Zulker
Try to keep all the basics of science subjects and also English
Zulker
Ya i know that I will do hard work but
Aakarsh
I have always found astronomy fascinating, but my first love is philosophy.
GARY
I wanna become astromer as neel armstrang
Aakarsh
that's an astronaut
Emilya
Neil Armstrong
antman
what is your name who are you where are you why are you there
GARY
Neil Armstrong was an astronaut. You first work is to know the meaning of astrophysicist, astronomer, astronaut, astrologer and to find the work of the individual. And the most important thing is to be good in English. Because u have done some mistakes while doing chat with me like :an astronomer
Zulker
sorry I'm talking to somebody else someplace else on the internet
GARY
The heading will be "How to become an astronomer"
Zulker
jus be yourself and follow your passion and I think there is no need to copy anyone
Aarushi
hey it's simple yaar, it's based on your curiosity
neehar
what is the length of Earth
comedy Reply
The Earth is not a perfect sphere, however it's roughly 25,000 miles, or 131.5 million feet, or 1.6 Billion inches. In the Metric system its HUGE!
Jocofranc
Thank for your knowledge.Sr
nyo
But can I also know and calculate the whole weight of earth includeing flora, funa,water on earth even air's weight.Depending on super compruter, I think it can be.For the very detail in the Metric System.Such as kilogram and pound.
nyo
Earth is about 6 Sextillion metric tons check out formula at ***science.howstuffworks.com/environmental/earth/geophysics/planet-earth-weigh.htm
Jocofranc
thank again Sr.I wanna ask another question if you don't mind.The earth had 4.5billion years age old.My question is from fire ball to today more and more stable geology was how happen.Please answer me starting of life on this planet and how many era are there and the earth history.
nyo
My friend you have asked a question(s) that scientists struggle with. There is no short cut...the information is lenghty. Here is an additional site for you. It is a good place to start. ***newscientist.com/article/dn17453-timeline-the-evolution-of-life/
Jocofranc
Sr,your web site is very useful to me.And another question is how gene or DNA transfer from parent to sibling.how we know the first DNA of our first human from arfarica.
nyo
Good morning Nyo; ur question is out of my league, although fascinating. Maybe you'll find this page helpful: ***genetics.thetech.org/ask/ask445
Jocofranc
If speed is so many thousand time of light and start to travel from one point,how speed will return to it starting point even in straight line back.Will time and energy be limit it?Can it be posible.Please let me explain Sr.
nyo
hey neo ur curiosity is great, where r u from?
neehar
in physics actully no but thory make possible
gdjm
earth is about 13,000 kilometers across
Hunter
there is aliens or parallel universe
Chithra Reply
There may be both who knows
Debabrata
It would be quite surprising to me to learn that there is no aliens in our very own galaxy. The size of our galaxy alone seems to favour life. There's so many reasons why we haven't made contact but as our technology grows we will undoubtedly find answers. I also think that it's possible that ...
Jocofranc
But I don't think that that alien species might be developed.... Because if they were developed or advanced than us then they must have located us first and might have even contacted us
Debabrata
many other universes exist. I don't think that they would be parallel universes in the purest sense of the word. Only speculations on my part. We simply dont know with certainty.
Jocofranc
Hi Debabrata; You may be right. It's hard to say with certainty. An ugly truth (guess really) is that Tellurian are simply not interesting. We are not all that!
Jocofranc
Yeah Jocofranc you are also right about parallel universe. Who knows what lies ahead.
Debabrata
the parallel universe theory is something secondary but first let's focus on the nearby objects like titan which shows great hope
Aarushi
I read it before about aliens, but I have to research on it
neehar
yeah of course
neehar
What about XTs... Exo-Tellurians. Ancient people who could have obtain a spurt of technology and left home. Progression of men r not always linear but sometime cyclic. Civilization comes and goes. Geo-upheaval can easily erase any evidence. Our bros r out there. (have fun 😁)
Jocofranc
sir jocofranc I think u didn't listen about aliens who send a message to our space research organization.
neehar
what is inside Jupiter how it will rain there
Chithra Reply
interesting question!
Mahesh
would love to learn about it 😃
Morrie
jupiter is a gas gaint so you can't walk on the surface like here on earth, but there is a solid core.
Yannick
The latest term being used is a 'fuzzy core'. An inbetween of solid and liquid. Check ***sciencemag.org/news/2019/08
Jocofranc
I think it based on the nature of the source in Jupiter
neehar
Scientists believe that onJupiter and Saturn, it rains diamonds. New atmospheric data for the gas giants indicates that carbon is abundant in its dazzling crystal form, the BBC reports. Lightning storms turn methane into soot which hardens into chunks of graphite and then diamond as it falls.
gdjm
Where is a moon HOW MANY KILLOMETER FAR FROM EARTH
Guddi Reply
On Average 383,000 Kilometers, Between Perigee And Apogee.....
Adam
386,000 km
neehar
3.6×10^5 km
gdjm
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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