# 26.3 Properties of galaxies

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## Learning objectives

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

• Describe the methods through which astronomers can estimate the mass of a galaxy
• Characterize each type of galaxy by its mass-to-light ratio

The technique for deriving the masses of galaxies is basically the same as that used to estimate the mass of the Sun, the stars, and our own Galaxy. We measure how fast objects in the outer regions of the galaxy are orbiting the center, and then we use this information along with Kepler’s third law to calculate how much mass is inside that orbit.

## Masses of galaxies

Astronomers can measure the rotation speed in spiral galaxies by obtaining spectra of either stars or gas, and looking for wavelength shifts produced by the Doppler effect    . Remember that the faster something is moving toward or away from us, the greater the shift of the lines in its spectrum. Kepler’s law, together with such observations of the part of the Andromeda galaxy that is bright in visible light, for example, show it to have a galactic mass of about 4 × 10 11 M Sun (enough material to make 400 billion stars like the Sun).

The total mass of the Andromeda galaxy is greater than this, however, because we have not included the mass of the material that lies beyond its visible edge. Fortunately, there is a handful of objects—such as isolated stars, star clusters, and satellite galaxies—beyond the visible edge that allows astronomers to estimate how much additional matter is hidden out there. Recent studies show that the amount of dark matter    beyond the visible edge of Andromeda may be as large as the mass of the bright portion of the galaxy. Indeed, using Kepler’s third law and the velocities of its satellite galaxies, the Andromeda galaxy is estimated to have a mass closer to 1.4 × 10 12 M Sun . The mass of the Milky Way Galaxy    is estimated to be 8.5 × 10 11 M Sun , and so our Milky Way is turning out to be somewhat smaller than Andromeda.

Elliptical galaxies do not rotate in a systematic way, so we cannot determine a rotational velocity; therefore, we must use a slightly different technique to measure their mass. Their stars are still orbiting the galactic center, but not in the organized way that characterizes spirals. Since elliptical galaxies contain stars that are billions of years old, we can assume that the galaxies themselves are not flying apart. Therefore, if we can measure the various speeds with which the stars are moving in their orbits around the center of the galaxy, we can calculate how much mass the galaxy must contain in order to hold the stars within it.

In practice, the spectrum of a galaxy is a composite of the spectra of its many stars, whose different motions produce different Doppler shifts (some red, some blue). The result is that the lines we observe from the entire galaxy contain the combination of many Doppler shifts. When some stars provide blueshifts and others provide redshifts, they create a wider or broader absorption or emission feature than would the same lines in a hypothetical galaxy in which the stars had no orbital motion. Astronomers call this phenomenon line broadening . The amount by which each line broadens indicates the range of speeds at which the stars are moving with respect to the center of the galaxy. The range of speeds depends, in turn, on the force of gravity that holds the stars within the galaxies. With information about the speeds, it is possible to calculate the mass of an elliptical galaxy.

what is the model about the earth being the center of the universe called?
geocentric model
SCIENCE
SCIENCE
heliocentric universe
Jason
you are wrong Jason skerbinc
SCIENCE
in heliocentric model sun is at the center of the universe
Tanmay
what is black hole and stellar black hole
does Millikan consider electron axial spin in his discovery of electrons mass using q/m ratio ? If No how it is possible that he found rest mass of electron (9×10^31 Kg) ?
how did light from up
Before there stars , there was matter and radiation . Before there were neutral atoms ,there was an ionized plasma ,and when that plasma froms neutral atoms , those allow the Universe to deliver the earliest light we see today .......
Apsara
how to know the distance of a star
What is time
Sourav
Technical Infinite Measurement of Evolution
Doc
not by the book but the simplicity of word play ....
Doc
to calculate the distance of a star we use parallax method (as far as my school has taught me).
Tanmay
in this method we see the star from one point and then after traveling some distance perpendicular to the distance of star we see it again. With this we can make a triangle and then it's median will give our distance
Tanmay
And time is just a way of defining when an event happened
Tanmay
oh and we will know the distance of all the 3 sides of triangle formed above if we multiply the time taken by light with speed of light and the third side will be known to us as it is dependent on our will
Tanmay
what are wormholes?
Neil
What is the difference between black hole and white hole ?
Apsara
black hole is the place where the massive body makes a large disturbance in space time fabric causing large gravity even light can't escape from it and all the bodies enter into on reaching a specific distance.
SCIENCE
A white hole is a hypothetical region of spacetime and singularity which cannot be entered from the outside, although energy-matter and light can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter and light can
SCIENCE
White holes appear in the theory of eternal black holes
SCIENCE
worm holes join 2 distant region of spacetime through which you can travel, covering large distance in short time
Tanmay
for example if you need to travel from A to B on a plane of paper then you can travel in a straight line joining those points but a worm hole will join those points using 3rd dimention which will make you travel that distance in short time
Tanmay
Hi,Do you known Kyaw Nyein Chan Hein!
No
Astrophysicist
Hello : I'm Kyaw Nyein Chan
zin
Hi I'm Kyaw Nyein Chan
Hello : good moring, I'm Kyaw Nyein Chan
hello good morning
Astrophysicist
good morning bro
Tanmay
how do you do?
Astrophysicist
I am fine what about you?
Tanmay
same as you
Astrophysicist
cool
Tanmay
How the Big Bang is made .
Kyaw Nyein Chan
zin
What is heliocentric
helio is from the greek god helios or ilios and means sun. Centric is from central. So heliocentric means with the sun in the middle
Robin
Usually in the context of astronomy its about the moment the world goes from the idea of 'earth in the middle of the solar system' to the idea that the sun is in the center of the solar system
Robin
so from a geocentric (geo meaning earth) to a heliocentric idea
Robin
yes correct this theory states that sun is in the middle of the solar system rather than earth
Tanmay
oh you have already said this
Tanmay
Hello: good afternoon
zin
hello
Tanmay
Thankyou to all to solve my doubt . I don't know more about the solar system beacause i am a small little girl my old is 9 and i love science but i am trying to learn more about science .
Apsara
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
Tanmay
You not alone Apsara I'm 14 but know little about the universe
Swazi
what is solar system
Himanshi
what do you mean by astronomy
Himanshi
astronomy is a study of planetary science.
Astronomy is a branch of Physics that deals with the study of celestial bodies.
Satyam
An astronomy is a scientist in the field of astronomy who focuses their studies on a specific questions or field outside the scope of Earth 🌍🌎🌏 . They observe astronomical objects such as star's , planets , moon , comets and galaxies - in either observational (by analyzing the data)
Apsara
what is the difference between astronomy and cosmology
Astrophysicist
The collection of eight planets and their moon's in orbit around the Sun , together with smaller bodies in the form of asteroids , meteoroids and comets . The planet of the solar system are ( in order of distance from the sun ) Mercury , Venues , Earth ,Mass , Jupiter , Sartune ,Uranus , Neptune .
Apsara
In Astronomy, we study about celestial objects but in cosmology we study the origin and the formation of universe.
Satyam
how much temperature were can find on neptune
the temperature is between -200 to -218 degree Celsius
Tanmay
how can I apply online astronomy course ? if you have link send it.
what is astronomy
Khanyisa
Does any body know about Quantum Battery ?
yea I tell you briefly callme
Eyiba
How to get into the field of astronomy after completing B.tech in department of ECE?
Well my option is to do your job while getting a distant education from a reptued university for B.Sc and try for M.Sc in IIA and remaining astrophysics universities in india
Mahesh
Do I require GATE score for that?
Lakshmi
Yeah I meañ get a better score in gate.I think the university considers it and would help you to direct enroll into a masters progarmme
Mahesh
But the score should be good.
Mahesh
okay. I will do my best.
Lakshmi
Thank you
Lakshmi
hi guys I am from Ethiopia.I don't know detailedly about astronomy and I want to be astronaut .can you help me guys?
Dawit
I know that astronauts have to be physically fit
Tanmay
I don't know anything else though
Tanmay
If you want to be a astronaut you should complete medicine.
Mahesh
I had done by my b.tech ece long back and then I started doing job...but I still want to switch my field to astronomy, I m confused how shall I pursue it..
Shina
what should we pursue after 12th std B.Sc program and further or through engineering?
Ganesh
Maybe you should ask this question in quora.Professors will tell you answer to the question.
Mahesh
ok
Ganesh