Properties of Main Sequence Stars

•Masses

•Luminosities

•Lifetimes

•Distribution

•

How do we know the masses of stars?

By studying binary stars!

“The mass of any central object can be determined if it has an observable orbiting satellite.”

Center of mass

Main Sequence Stars

y = 0.0151x6 - 0.4107x5 + 4.5331x4 - 25.9297x3 + 80.9918x2 - 131.3942x + 88.6098

R2 = 0.9980

0.1

1.0

10.0

100.0

Spectral Type

Ma

ss

in

So

lar

Un

its

O B A F G K M

Stars have a fairly narrow range of masses.

Main Sequence Stars

y = 0.0151x6 - 0.4107x5 + 4.5331x4 - 25.9297x3 + 80.9918x2 - 131.3942x + 88.6098

R2 = 0.9980

0.1

1.0

10.0

100.0

Spectral Type

Ma

ss

in

So

lar

Un

its

O B A F G K M

Stars have a fairly narrow range of masses.

Sun-like stars are 1 solar mass

Main Sequence Stars

y = 0.0151x6 - 0.4107x5 + 4.5331x4 - 25.9297x3 + 80.9918x2 - 131.3942x + 88.6098

R2 = 0.9980

0.1

1.0

10.0

100.0

Spectral Type

Ma

ss

in

So

lar

Un

its

O B A F G K M

Stars have a fairly narrow range of masses.

Sun-like stars are 1 solar mass

O & B stars are a few 10’s of solar masses

Main Sequence Stars

y = 0.0151x6 - 0.4107x5 + 4.5331x4 - 25.9297x3 + 80.9918x2 - 131.3942x + 88.6098

R2 = 0.9980

0.1

1.0

10.0

100.0

Spectral Type

Ma

ss

in

So

lar

Un

its

O B A F G K M

Stars have a fairly narrow range of masses.O & B stars are a few 10’s of solar masses

M stars are a few 10’ths of a solar mass

Sun-like stars are 1 solar mass

How do we know the luminosity of stars?

1. From direct measurement of a star’s distance through stellar parallax and its apparent magnitude we can calculate the absolute magnitude: M = m - 5log(D) + 5

2. From theoretical nuclear fusion calculations the luminosity can be modeled as a function of spectral type with reasonable accuracy. (After all we make the best nuclear bombs!)

Main Sequence Stars

y = 0.0347x5 - 0.6105x4 + 4.23x3 - 14.575x2 + 26.982x - 20.282

R2 = 0.9871

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Spectral Type

Ab

solu

te M

agn

itu

de

O B A F G K M

Stars have a HUGE range of luminosities.

Main Sequence Stars

y = 0.0347x5 - 0.6105x4 + 4.23x3 - 14.575x2 + 26.982x - 20.282

R2 = 0.9871

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Spectral Type

Ab

solu

te M

agn

itu

de

O B A F G K M

Stars have a HUGE range of luminosities.

Sun-like stars are 1 solar luminosity

Main Sequence Stars

y = 0.0347x5 - 0.6105x4 + 4.23x3 - 14.575x2 + 26.982x - 20.282

R2 = 0.9871

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Spectral Type

Ab

solu

te M

agn

itu

de

O B A F G K M

Stars have a HUGE range of luminosities.

O & B stars are 10,000

solar luminosities

Sun-like stars are 1 solar luminosity

Main Sequence Stars

y = 0.0347x5 - 0.6105x4 + 4.23x3 - 14.575x2 + 26.982x - 20.282

R2 = 0.9871

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Spectral Type

Ab

solu

te M

agn

itu

de

O B A F G K M

Stars have a HUGE range of luminosities.

O & B stars are 10,000

solar luminosities

M stars are a few 10,000’ths of a solar luminosity

Sun-like stars are 1 solar luminosity

How do we know stellar lifetimes?1. Again, theoretical nuclear modeling yields

estimates of main sequence lifetimes.

2. However, a simple estimate can be made based upon the mass of the star and its luminosity as shown below.

Spectral Type

Mass

M

Luminosity

L

Lifetime in Solar Lifetimes

O stars 30 30,000 30/30,000 = .001

M stars .3 .0003 .3/.0003 = 1,000

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

A few Million Years for O stars

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

A few Million Years for O stars

A few Trillion

Years for M stars

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

A few Million Years for O stars

A few 10’s of Trillion

Years for M stars

The Age of the Universe is now

considered to be 13.8 Billion years ± 1%

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

A few Million Years for O stars

A few 10’s of Trillion

Years for M stars

The Age of the Universe is now

considered to be 13.8 Billion years ± 1%

None of the first O through G stars exist anymore.

Main Sequence Stars

y = 8E+07x3.3742

R2 = 0.9873

y = 1E+07x5.2587

R2 = 0.9675

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

Spectral Type

Lif

etim

e in

Yea

rs

O B A F G K M

Stars also have a HUGE range of

lifetimes.

10 Billion Years for Sun-like stars

A few Million Years for O stars

A few 10’s of Trillion

Years for M stars

The Age of the Universe is now

considered to be 13.8 Billion years ± 1%

Every M star ever created is still a main sequence M star.

The HUGE range in stellar lifetimes is reflected in the

distribution of stars in the Milky Way Galaxy.

• Long lived K & M stars are most abundant.• Short lived O & B stars are very rare.• Other very luminous stars (Giants) will also be rare due to

their short lifetimes. • White dwarf stellar remnants have VERY long cooling

times but are very low luminosity. They may represent the most common object in the galaxy but are undercounted.

Note the high abundance of

M dwarfs.

Note the low abundance of O & B stars.

There are also very few giant

stars.

There are probably many

more white dwarf stellar remnants

that are too dim to see.

Number of Stellar Types in the Milky Way

0% 1% 0% 0% 1% 3%7%

13%

68%

9%

0%

20%

40%

60%

80%

O-M F-M O B A F G K M B-F

Supergiant(I & II)

Red Giant(III)

Main Sequence (V) WhiteDwarf

Num

ber

Number of Stellar Types in the Milky Way

0% 1% 0% 0% 1% 3%7%

13%

68%

9%

0%

20%

40%

60%

80%

O-M F-M O B A F G K M B-F

Supergiant(I & II)

Red Giant(III)

Main Sequence (V) WhiteDwarf

Num

ber

Only 7% of the stars in the Milky Way galaxy are Sun-like and half of them are in

multiple star systems. Thus, star and planetary systems like our own represent less than 3% of all the stars in

the galaxy.

Number of Stellar Types in the Milky Way

0% 1% 0% 0% 1% 3%7%

13%

68%

9%

0%

20%

40%

60%

80%

O-M F-M O B A F G K M B-F

Supergiant(I & II)

Red Giant(III)

Main Sequence (V) WhiteDwarf

Num

ber

The most common stars in the Milky Way Galaxy are of such low luminosity that they are difficult to see even when they are right “next

door”. These stars are utterly invisible in distant

galaxies.

Number of Stellar Types in the Milky Way

0% 1% 0% 0% 1% 3%7%

13%

68%

9%

0%

20%

40%

60%

80%

O-M F-M O B A F G K M B-F

Supergiant(I & II)

Red Giant(III)

Main Sequence (V) WhiteDwarf

Num

ber

The very rare short lived stars are the most luminous and are the easiest seen in the night sky and in other

distant galaxies

This Spiral Galaxy resembles our Milky Way Galaxy

This Spiral Galaxy resembles our Milky Way Galaxy

This is a more distant galaxy of a different type.

Hot O and B stars dominate

the luminosity in the Spiral Arms There are

very many low

luminosity K and M

stars in the apparently

empty space

between the spiral arms.

• In the following M101 galaxy images you will see the galaxy through a red filter and then through a blue filter. (Repeats once)

• Notice that the galaxy is dimmer and less defined in red light and brighter and sharper in blue light.

Only the most luminous stars show up distinctly in distance galaxies. The most

common stars in the galaxy are individually invisible.

Red Filter

Spiral Galaxy

M101

Blue Filter

Spiral Galaxy

M101

Red Filter

Spiral Galaxy

M101

Blue Filter

Spiral Galaxy

M101

The Milky Way

0

25

50

75

100

O-M F-M O B A F G K M B-F

Supergiant(I & II)

Red Giant(III)

Main Sequence (V) WhiteDwarf

Luminosity Class and Spectral Type

Percentage of Galactic LuminosityPercentage in Number Percentage of Galactic Stellar Mass

75% of the Milky Way’s luminosity

arise from the rarest stars.

K & M stars account for ¾’s of the stars in the galaxy but contribute less than 5% of its

luminosity.

The properties of O and M Main Sequence Stars

O Stars• Mass 25 M

• Radius 6 R

• Temperature 30,000K• Luminosity 80,000 L

• M -6• Main Sequence Lifetime

3 Million years

• Mass 0.1 M

• Radius 0.1 R

• Temperature 3,000K• Luminosity 0.0001 L

• M 15• Main Sequence Lifetime

10 Trillion years

M Stars

For the

Test