The Hertzsprung-Russell Diagram Notes

What is the Hertzsprung-Russell Diagram?

A graphical tool plotting luminosity against surface temperature

What does the Hertzsprung-Russell Diagram organize?

stars based on spectral type and luminosity

The Hertzsprung-Russell diagram (H-R diagram) plots

luminosity or absolute magnitude against temperature or spectral type.

What does the vertical axis represent?

luminosity, absolute magnitude, or intrinsic brightness

Where do more luminous stars appear?

Toward the top

Where do dimmer stars appear?

Toward the bottom

What does the horizontal axis represent?

represents temperature, spectral type, or color

Where are hotter, bluer stars plotted?

To the left

Where are cooler, redder stars plotted?

To the right

The spectral types O, B, A, F, G, K, and M are typically labeled

along this axis

The construction of the H-R diagram relies on determining

the absolute magnitude of stars, which requires knowing their distances

These stars are categorized as main sequence stars, representing a

stable phase in a star's life cycle

What do hotter main sequence stars look like?

More luminous

What do cooler main sequence stars look like?

Less luminous

Main sequence stars can be considered

normal stars

Main Sequence Stars are assigned the luminosity type

Roman numeral V

The spectral-luminosity type G2V indicates

a yellow-hot star on the main sequence

Where do most stars fall on the Hertzsprung-Russell (H-R) diagram?

Most stars fall along the main sequence of the H-R diagram.

Where are the cool but intrinsically bright stars located on the H-R diagram?

In the upper right corner.

Why is it surprising that some stars are both cool and bright?

Because cooler stars are expected to be dimmer, so being both cool and intrinsically bright seems contradictory.

What common misunderstanding do people have about bright stars on the H-R diagram?

That they appear bright only because they are close to Earth.

Which physical law helps explain how a star can be cool but still bright?

The Stefan-Boltzmann law.

According to the Stefan-Boltzmann law, what factors determine a star's luminosity?

A star's surface area (which depends on radius) and temperature to the fourth power.

How can a star be cool but still have high luminosity?

If the star has a very large radius, it can emit a large total amount of energy despite a low surface temperature.

What two factors determine a star's luminosity according to the Stefan-Boltzmann law?

Radius and temperature.

How can a cool star still be very luminous?

If the star has a very large radius, it can emit a lot of energy despite being cool.

Why are stars in the upper right of the H-R diagram luminous?

Because they are large and cool

What are large, cool, and luminous stars in the upper right of the H-R diagram called?

Red giants

What are the four luminosity types of red giants, from most to least luminous?

Red supergiants (Type I), red bright giants (Type II), red ordinary giants (Type III), and red subgiants (Type IV)

Which type of red giant is the most luminous and largest?

Red supergiants (Type I)

How large can red supergiants get?

Their radii can be comparable to Earth's orbit and large enough to engulf the inner Solar System

What type of red giant is the least luminous and smallest?

Red subgiants (Type IV)

Red Giants Characteristics

Cool (towards the right on the diagram) and intrinsically bright (towards the top)

Where are white dwarfs located on the Hertzsprung-Russell (H-R) diagram?

In the lower left corner

What are the characteristics of white dwarfs on the H-R diagram?

They are hot and intrinsically dim

Why are white dwarfs intrinsically dim despite being hot?

Because they are very small in size

What is the approximate size of a white dwarf?

Roughly the same radius as Earth

What are the smallest types of stars?

White dwarfs

Main Sequence Stars

Band extending from the upper left to the lower right corner of the Hertzsprung-Russell diagram

Hotter Main Sequence stars are __________ than colder Main Sequence stars

more luminous (intrinsically brighter)

Cooler main sequence stars are

less luminous (less intrinsically dimmer)

Most stars, including the Sun, are located on the

main sequence

Which star is more luminous: the M1 main sequence or the M8 main sequence?

M1 main sequence

Which star is larger: F0 or F5?

Not enough information is given

Which star is larger: F0V or F5V

F0V

Which star is smaller: B2 or B7?

Not enough information is given

Which star is smaller: B2V or B7V?

B7V

Which star is larger: M3 or M9?

Not enough information is given

Which star is larger: M3V or M9V?

M3V

Which star is more massive: G3V or G6V?

G3V

Which star is less massive: O4V or O6V?

O6V

Which star is more massive: M2 or M4?

Not enough information is given

Which star is more massive: M2V or M4V?

M2V

Which star is more abundant: B3V or B8V?

B8V

Which star is less abundant: K2V or K6V?

K2V

Which star lives longer in the main sequence: A2 or A6?

A6

Which star lives shorter in the main sequence: K3 or K7?

K3

Which star, when they were main-sequence stars, was hotter: the outer or inner star?

The outer star died first.

What is the Sun intermediate in compared to other main sequence stars?

Temperature, size, mass, age, and luminosity

Stars early in the OBAFGKM sequence

will be hotter, more luminous, larger, and more massive

Stars late in the OBAFGKM sequence

will be cooler, less luminous, smaller, and less massive

Is the main sequence an evolutionary sequence for stars?

No, stars do not evolve along the main sequence from one type to another.

Early-type stars

stars towards the hotter end of the main sequence (OBA)

Late-type stars

stars are those towards the cooler end (FGKM)