chapter 21 stellar classification

Chapter 21

Stellar

Classification

A Few Definitions

1. M☉ a solar mass, based upon our sun’s mass.

a. 1.5 M☉ is 1.5 times the mass of the sun.

b. 12 M☉ is 12 times the mass of the sun

2. L☉ is the luminosity of a star, based upon our sun’s

luminosity

a. 2.5 L☉ is 2.5 times the luminosity of the sun.

b. 1,000 L☉ is 1,000 times the luminosity of the sun

3. R☉ is the radius of a star, based on the Sun’s radius.

a. 0.5 R☉ is 0.5 times the luminosity of the sun.

b. 10 R☉ is 10 times the luminosity of the sun

Categorizing Stars

There are several different ways to categorize stars:

1. Size

Categorizing Stars


1. Size

2. Color

Categorizing Stars


1. Size

2. Color

3. Temperature

Categorizing Stars


1. Size

2. Color

3. Temperature

4. Luminosity

Categorizing Stars


1. Size

2. Color

3. Temperature

4. Luminosity

5. Lifespan

UY Scuti is a red supergiant and pulsating variable star in the constellation Scutum. It is

a current and leading candidate for being the largest known star by radius and is also

one of the most luminous of its kind. It has an estimated radius of 1,708 solar radii

(1.188×109 kilometres; 7.94 astronomical units); thus a volume nearly 5 billion times that

of the Sun. It is approximately 2.9 kiloparsecs (9,500 light-years) from Earth. If placed at

the center of the Solar System, its photosphere would at least engulf the orbit of Jupiter.

In the 1890s, many scientists were interested in developing

a classification scheme for the stars.

Categorizing Stars

Edward C. Pickering at Harvard University, together with

his assistant Williamina P. Fleming, assigned stars a letter

according to how much Hydrogen could be observed in

their spectra.

Categorizing Stars

• Stars labeled A had the most Hydrogen, B the next most,

and so on through the alphabet. There were 22 types in

all.

• This scheme was rather cumbersome, and it wasn't clear

what its physical significance was.

Categorizing Stars

Classification Comment

A, B, C, D Hydrogen lines dominant.

E, F, G, H, I, K, L

M

N Did not appear in the catalogue.

OWolf–Rayet spectra with bright

lines.

P Planetary nebulae.

• In 1901, another of Pickering's

assistants, Annie Jump

Cannon, also began to work

on the classification sequence.

• Her meticulous observations

led her to simplify the 22-type

scheme into a sequence of

temperature: O B A F G K M.

Categorizing Stars

Spectral Classification of Stars

Oh Be A Fine Girl Kiss Me

Oh Be A Fine Guy Kiss Me

Oh Bother, Another F's Gonna Kill Me

Only Bizarre Astronomers Find Gratitude Knowing Mnemonics.


The scheme is based on lines which are mainly sensitive to stellar surface

temperatures rather than actual compositional differences, gravity, or

luminosity.

Spectral Classification of Stars• Even though the atmosphere of a star is mostly hydrogen the small traces

of the other elements all contribute to producing the spectrum for the star.

• Various atoms and molecules contribute to the formation of spectral lines as

a function of temperature.

Spectral Lines of Stars

• Some stars have strong Hydrogen lines, while others don’t (B

vs. G)

• Some stars have very strong lines from molecules of NaI, TiO

(B-K vs. M)


O B A F G K M and more.

• Each spectral type is divided into 10 subclasses, A0, A1, A2,

...A9 etc.

• The spectral types and sub-classes represent a temperature

sequence, from hotter (O stars) to cooler (M stars), and from

hotter (subclass 0) to cooler (subclass 9).

• The temperature defines the star's "color" and surface

brightness.

K3K4 K2 K1 K0 G9 G8 G7


• The Harvard classification only takes into account the effect of

the temperature on the spectrum.

• It is found that two stars with the same effective temperature

may have widely different luminosities.

• Thus for a more precise classification, one has to take into

account the luminosity of the star.

• A new classification is introduced by Morgan, Keenan and

Kellman (known as MKK) called Yerkes spectral classification.

• In this classification six different luminosity classes are

proposed.

Luminosity Classification

Yerkes Luminosity Classes

Class Description

0hypergiants or extremely luminous

supergiants

Ia luminous supergiants

Iab intermediate-size luminous supergiants

Ib less luminous supergiants

II bright giants

III normal giants

IV subgiants

V main-sequence stars (dwarfs)

sd sub-dwarf

d dwarf

Main Sequence Stars

Lum

ino

sity

(re

lati

ve t

o S

un

)

1

100

10,000

0.01

0.0001

Temperature (Kelvin)

25,000 10,000 7,000 5,000 3,000

We start by drawing the axes:•Luminosity up the vertical axis (measured relative to the Sun)•Temperature along the horizontal axis (measured in Kelvin)

Where would you mark the Sun on the plot?•It has Luminosity of 1 relative to itself•Its temperature is 5800 K

The stars Vega and Sirius are brighter than

the Sun, and also hotter. Where would you

put them?

Some stars are much cooler and less luminous,

such as the closest star to the Sun, Proxima

Centauri. Where would you plot these?

These stars are called red dwarfs.

Sun

Sirius

Vega

Proxima

Centauri

In fact, most stars can be found somewhere along a line in this graph.

This is called the “Main Sequence”.

Main Sequence Stars

• Main sequence stars fuse hydrogen atoms to form helium

atoms in their cores.

• About 90 percent of the stars in the universe, including the

sun, are main sequence stars.

• These stars can range from about a tenth of the mass of the

sun to up to 200 times as massive.

Class O Main Sequence Stars

Class O main sequence stars are rare objects; it is

estimated that there are no more than 20,000 class O stars

in the entire Milky Way, around one in 10,000,000 of all

stars.

• These stars are

between 15 and 90 M☉

and have surface

temperatures between

30,000 and 50,000 K.

• Most of their output is

in the ultraviolet range

giving the stars a

bluish-white color.

HDE 226868 is a type O9.7

binary with Cygnus X-1, a

black hole, as its partner.


• Class O stars

are very young,

no more than a

few million

years old, and in

our galaxy they

all have high

metallicities,

around twice

that of the sun.

• Almost all are

fated to end

their brief lives

in spectacular

supernova

explosions.

The Trifid Nebula (M20) is sculpted and lit

by the luminous O 7.5 star visible at its

center in this infrared image.


• Their luminosities are

between 30,000 and

1,000,000 L☉.

• Their radii are more modest

at around 10 R☉.

• Their stellar winds have a

terminal velocity around

2,000 km/s (the sun’s 400

km/s).

Zeta Puppis is an O4 star in the

constellation of Puppis


• Their luminosities are

between 30,000 and

1,000,000 L☉.

• Their radii are more modest

at around 10 R☉.

• Their stellar winds have a

terminal velocity around

2,000 km/s (the sun’s 400

km/s).

AE Aurigae, known also as the

Flaming Star, is a type O9.5V


A B Class star is a large, luminous, blue-white star.

Class B Main Sequence Stars

Often they are found together with O stars in OB

associations since, being massive, they are short-lived and

therefore do not survive long enough to move far from the

place where they were formed.


NGC 4755:The Jewel Box can be seen in the southern

constellation of the cross (Crux).

Their brief main sequence

careers, measured in tens

of millions of years,

probably allows too little

time for even the most

primitive forms of life to

develop on any worlds

that circle around them.


Rigel, or Beta Orionis, a type B8

About 1 in 800 (0.125%) of the main-sequence stars in the

solar neighborhood are B-type main-sequence stars.


Spica is a type is B1 III-IV in the constellation Virgo.

• B class stars have a surface temperatures between

10,000 and 30,000 K.

• B-type star may have a mass in the range 2 to 16 M☉

and a luminosity of 25-30,000 L☉


Part of the constellation of Carina, Epsilon Carinae is an example of a

double star featuring a main-sequence B2V type star

They

have a

radius of

2 to 7 R☉


Alnilam, or Epsilon Orionis is type B0Iab,

and is the middle star in Orion's belt.

• A-type stars are among the more common naked eye stars,

and are white or bluish-white.

• Approximately 1 out of every 167 stars is a Class A star.

Class A Main Sequence Stars

They have masses of around 1.4 to 2.1 times the mass of the Sun, and

surface temperatures anywhere from 7112 K to 11500 K.


Altair in the Constellation: Aquila

They have a radius

of 1.4 – 1.8 R.


• They have a radius of 1.4 – 1.8 R.

• Their luminosity 5-25 L


An artist's impression of Sirius A and Sirius

B, a binary star system

They exist on the

main-sequence

for a mere 400

million years.


Fomalhaut is an A3 main-sequence star in

Piscis Austrinus

• An F-type star is a main-sequence, hydrogen-fusing star.

• About 1 in 33 (3.03%) of the main-sequence stars in the solar

neighborhood are F-type stars

Class F Main Sequence Stars

• These stars have from 1.0 to 1.4 times the mass of the Sun

and surface temperatures between 6,000 and 7,600 K.

• This temperature range gives the F-type stars a yellow-white

hue.

• The main-sequence star is referred to as a yellow-white

dwarf star.


Artistic renditions of F Type stars

Because of their size some F Type stars explode as supernovae

when they die and become a neutron star. But most of them will

become a white dwarf.


Star KIC 8462852 (also Tabby's Star or Boyajian's Star) is an F-type

main-sequence star located in the constellation Cygnus

Infrared Ultraviolet

Class F Main Sequence Stars F-Type stars live an average of 5 billion years.

Procyon also designated Alpha Canis Minoris is the brightest star in the

constellation of Canis Minor.

The radius is 1.15-1.4 R ☉ with a luminosity of 1.5-5L☉.


Gamma Virginis is a binary star system in the constellation of

Virgo.

Class G Main Sequence Stars Class G main-sequence stars make up about 7.5%, nearly one

in thirteen, of the main-sequence stars in the solar

neighborhood.

Class G Main Sequence Stars

As many as 512 or

more stars of spectral

type "G" are currently

believed to be

located within 100

light-years or (or 30.7

parsecs) of Sol --

including Sol itself.


• A G-type main-

sequence star are

often called a

yellow dwarf stars

or G dwarf stars.

• Although the term

"dwarf" is used to

contrast yellow

main-sequence

stars from giant

stars, yellow

dwarfs like the Sun

outshine 90% of

the stars in the

Milky Way.

Epsilon Geminorum is a G8 star in the constellation

of Gemini


These stars have about

0.84 to 1.15 solar

masses and a radius of

0.96-1.14 R ☉

61 Virginis is a G7V star in Virgo.


Not too surprisingly,

the L☉ is 0.6-1.5.

HD 80606 and HD 80607 are two G5 stars

comprising a binary star system


• Surface

temperature of

between 5,300 and

6,000 K.

• The lifespan is

approximately 10

billions years.


Epsilon Virginis also named Vindemiatrix, is a star in the zodiac

constellation of Virgo.

A G-type main-sequence star is converting the element

hydrogen to helium in its core by means of nuclear fusion.

Class G Main Sequence Stars Many G type stars have planets. They include the Sun, 61

Virginis, HD 102365, HD 147513, 47 Ursae Majoris, Mu Arae,

Tau Ceti and Alpha Centauri.

The two bright stars are (left) Alpha

Centauri and (right) Beta Centauri.

Class G Main Sequence Stars A super Earth orbits a yellow Sun-like star. This sizzling-hot

world takes only 8 hours and 54 minutes to orbit its star, a

journey which takes Mercury 88 days.

The G-type dwarf star EPIC 228732031.


• Mu Arae is a main sequence G-type star approximately 50

light-years away from the Sun in the constellation of Ara.

• The star has a planetary system with four known extrasolar

planets (designated Mu Arae b, c, d and e), three of them

with masses comparable to that of Jupiter.

• The system's innermost planet was the first 'hot Neptune' or

'super-Earth' to be discovered.

Class K Main Sequence Stars Orange-red "K" dwarf stars are more common than brighter

OBAFG stars but most are not visible in Earth's night despite

their relative abundance.

Class K Main Sequence Stars Roughly a thousand stars (947+) of spectral type "K" have been

tentatively identified and located within 100 light-years (ly) of

Sol, but only 155 within 50 ly.

Class K Main Sequence Stars Class K star are typically orange dwarfs.

Epsilon Eridani, the

bright star at left

center of meteor, is a

orange-red K-type

star

Class K Main Sequence Stars They are smaller than Sol, 0.7-0.96 R☉ with a mass of 0.45-

0.8 M☉.

Epsilon Indi is an orange dwarf star approximately just 12

light-years away in the constellation of Indus.

HD 131399 C

is a K-type

main-sequence

star.

Class K Main Sequence Stars

HD 131399 C is a K-type main-sequence

star.

• With a

temperature

range of 3,700 –

5,200 K, the

luminosity

ranges 0.08-0.6

L☉.

• They have a long

lifespan of

approximately 15

billion years.

Class K Main Sequence Stars These stars are of particular interest in the search for

extraterrestrial life because they are stable on the main

sequence for a very long time (15 to 30 billion years, compared

to 10 billion for the Sun).

Artist's impression of the disk surrounding GG Tauri A

Class K Main Sequence Stars

These stars are of

particular interest in the

search for extraterrestrial

life because they are

stable on the main

sequence for a very long

time (15 to 30 billion

years, compared to 10

billion for the Sun).

54 Piscium is an orange dwarf star

approximately 36 light-years away in the

constellation of Pisces.

Class M Main Sequence Stars Over 76% of all stars are class M stars.

Class M Main Sequence Stars More than two thousand (2,026+) of spectral type "M" had been

tentatively identified and estimated to be within 100 light-years of

Sol.

Class M Main Sequence Stars A red dwarf is a small and relatively cool star on the main

sequence of M spectral type.

Alpha Centauri A and B are the bright stars; Proxima Centauri, a red dwarf

star, is the small, faint one circled in red.

Class M Main Sequence Stars Red dwarfs range in mass from a low of 0.08 to about 0.45 solar

mass and have a surface temperature of less than 3,700 K.

Wolf 359 is a M6.5V red dwarf star located

in the constellation Leo, near the ecliptic.

Class M Main Sequence Stars The have a luminosity less than 0.08 L☉

Barnard's Star is a M4V very-low-mass red dwarf in

the constellation of Ophiuchus.

Class M Main Sequence Stars

These stars are

expected to be

extremely long lived

stars…around 100

billion years (the age

of the universe is13.8

billion years).

EZ Aquarii is a triple star system all three

components are M-type red dwarfs.

Class Color

Fraction

of all

main-

sequence

stars (%)

Lifespan

(years)

Main-

sequence

mass

(M☉)

Main-

sequence

radius

(R☉)

Main-

sequence

luminosity

(L☉)

Effective

Temp.

(K)

O blue ~0.000032-8

million≥ 16 ≥ 6.6 ≥ 30,000 ≥ 30,000

Bblue

white0.13

10-100

million2.1–16 1.8–6.6

25–

30,000

10,000 –

30,000

A white 0.6400

million1.4–2.1 1.4–1.8 5–25

7,500 –

10,000

Fyellow

white3 5 billion 1.04–1.4 1.15–1.4 1.5–5

6,000 –

7,500

G yellow 7.6 10 billion 0.8–1.040.96–

1.150.6–1.5

5,200 –

6,000

K orange 12.1 15 billion 0.45–0.8 0.7–0.96 0.08–0.63,700 –

5,200

M red 76.45100

billion0.08–0.45 ≤ 0.7 ≤ 0.08

2,400 –

3,700


Class M76.45%

Class K12.1%

Class G7.6%

Class F3%Class A

0.6%

Class B0.13%

Class O0.00003%

Lum

ino

sity

(re

lati

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un

)

1

100

10,000

0.01

0.0001


25,000 10,000 7,000 5,000 3,000

Sun

Sirius

Vega

ProximaCentauri

Rigel

Betelgeuse

Deneb

Arcturus

Aldebaran

But not all stars lie on the main sequence. Some, such as Arcturus and Aldebaran, are much brighter than the Sun, but cooler. Where would these lie on the diagram?

These are orange giant stars.

The bright star Betelgeuse is even more luminous than Aldebaran, but has a cooler surface.

This makes it a red supergiant.

Even brighter than Betelgeuse are stars like Deneb and Rigel, which are much hotter.

These are blue supergiants.

Red Giant Stars• Giant stars have radii up to a few hundred times the Sun and

luminosities between 10 and a few thousand times that of the

Sun.

• The most common red giants are stars on the red-giant

branch that are still fusing hydrogen into helium in a shell

surrounding an inert helium core.

Red Giant Stars

• Betelgeuse, a M1–M2 Ia–ab star, has no known orbital

companions, so its mass cannot be calculated by that direct

method.

• Model fitting to evolutionary tracks give a current mass of

19.4–19.7 M☉, from an initial mass of 20 M☉

Red Giant Stars• Antares is on average the fifteenth-brightest star in the night

sky (M1.5Iab-Ib) and the brightest star in the constellation of

Scorpius.

• The brightness of Antares at visual wavelengths is about

10,000 times that of the Sun.

• The mass of the star has been calculated to be

approximately 11 to 14.3 M☉

Blue Giant Stars

A blue giant is a

hot star with a

luminosity class

of III (giant) or II

(bright giant).

The four brightest stars are blue supergiant stars,

with a red supergiant star at the center.

Blue Giant Stars

A blue giant is a hot star with a luminosity class of III (giant) or II

(bright giant).

Blue Giant Stars

Bellatrix, a B2III star, is the third-brightest star in the constellation of Orion,

5° west of Betelgeuse.

Blue Giant Stars

Alcyone, a B3III star, is the brightest star in

the Pleiades open cluster, which is a young

cluster, around 100 million years old.

Variable Stars• A variable star is a star whose brightness as seen from Earth

(its apparent magnitude) fluctuates.

• Many, possibly most, stars have at least some variation in

luminosity: the energy output of our Sun, for example, varies

by about 0.1% over an 11-year solar cycle.

Variable Stars• This variation may be caused by a change in emitted light or

by something partly blocking the light, so variable stars are

classified as either:

• Intrinsic variables, whose luminosity actually changes; for

example, because the star periodically swells and

shrinks.

Variable Stars• This variation may be caused by a change in emitted light or

by something partly blocking the light, so variable stars are

classified as either:

• Extrinsic variables, whose apparent changes in

brightness are due to changes in the amount of their light

that can reach Earth; for example, because the star has

an orbiting companion that sometimes eclipses it.

Cepheid Stars and Distance

Approximately 100 million ly across

RS Puppis is a

Cepheid variable

star in the

constellation of

Puppis.

Cepheid Variable Stars

Delta Cephei is a

quadruple star system

approximately 887 light-

years away in the northern

circumpolar constellation of

Cepheus, the King.

• At this distance, the visual

magnitude of the star is

diminished by 0.23 as a

result of extinction caused

by gas and dust along the

line of sight.

• It is the prototype of the

Cepheid variable stars that

undergo periodic changes

in luminosity.


Cepheid Stars and DistanceA Cepheid variable is a type of star that pulsates radially, varying

in both diameter and temperature and producing changes in

brightness with a well-defined stable period and amplitude.

Cepheid Stars and DistanceA strong direct relationship between a Cepheid variable's

luminosity and pulsation period established Cepheids as

important indicators of cosmic benchmarks for scaling galactic

and extragalactic distances.


This robust characteristic of classical Cepheids was discovered

in 1908 by Henrietta Swan Leavitt after studying thousands of

variable stars in the Magellanic Clouds.


This discovery allows one to know the true luminosity of a

Cepheid by simply observing its pulsation period.

Partial He ionization zone is opaque and

absorbs more energy than necessary to

balance the weight from higher layers.

=> Expansion

Upon expansion,

partial He ionization

zone becomes more

transparent, absorbs

less energy => weight

from higher layers

pushes it back inward.

=> Contraction.

Upon compression, partial He ionization zone

becomes more opaque again, absorbs more

energy than needed for equilibrium => Expansion


Cepheid Stars and DistanceThis in turn allows one to determine the distance to the star, by

comparing its known luminosity to its observed brightness.


Cepheid

variables can be

used to measure

distances from

about 1kpc to 50

Mpc

(1,000 pc –

50,000,000 pc).

Approximately 100 million ly across

Cepheid Stars and DistanceThen its absolute magnitude and apparent magnitude can be

related by the distance modulus equation, and its distance

can be determined.

m = apparent magnitude

M = absolute magnitude

r = distance

Cepheid Stars and DistanceFor example, if an astronomer observed a Cepheid star with

period of 34 days, comparing to previously measured Cepheids,

its absolute magnitude is -5.65. If its apparent magnitude was

+23.0, the astronomer could use the distance modulus equation:

m - M = 5 log d - 5

rearranged:

d = 10(m - M + 5)/5 parsecs

to find the distance to the Cepheid:

d = 10(23 – (-5.65) + 5)/5 parsecs

d = 106.73 parsecs

d = 5.4 × 106 parsecs

Comparison of Stellar Cores

Red Dwarf, 41.0%

Dwarf, 25.0%

0.5-1.0 Solar Masses, 19.0%

1-2 Solar Masses, 8.0%2-4 Solar

Masses, 3.0%

4-8 Solar Masses, 0.8%

Supergiants, 0.4%

Stars

Lum

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(re

lati

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un

)

1

100

10,000

0.01

0.0001


25,000 10,000 7,000 5,000 3,000

Sun

Sirius

Vega

Proxima

Centauri

Rigel

Betelgeuse

Deneb

Arcturus

Aldebaran

But not all stars lie on the main sequence.

Some, such as Arcturus and Aldebaran,

are much brighter than the Sun, but

cooler. Where would these lie on the

diagram?

These are orange giant stars.

The bright star Betelgeuse is even more

luminous than Aldebaran, but has a cooler

surface.

This makes it a red supergiant.

Even brighter than Betelgeuse

are stars like Deneb and Rigel,

which are much hotter.

These are blue supergiants.

Exotic star

Extreme helium star

Flare star

FU Orionis star

Helium star

Herbig Ae/Be star

Hydrogen-deficient star

Hypergiant

Intergalactic star

Iron star

Lambda Boötis star

Lambda Eridani variable

Lead star

Luminous blue variable

Mercury-manganese star

OB star

Chemically peculiar star

PG 1159 star

Photometric-standard star

Planck star

Stars, stars, and more stars

This is a list of most

of the known

TYPES of stars.

This list is not

complete.

Just a scratch

All roads

lead to where

stars end.

chapter 21 stellar classification

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