Lecture 12 & 13: Stellar Properties and Classification Overview

High mass star

A star that has a greater mass than low mass stars and evolves differently.

Low mass star

A star that has a lesser mass than high mass stars and evolves differently.

Planetary Nebula

The end stage of low

Planetary Nebulae

The plural form of planetary nebula.

Nova

A small explosion on a stellar surface that causes a star to suddenly become bright.

Novae

The plural form of nova.

Binary star

Two stars orbiting around each other.

Supernova

A powerful and luminous explosion of a star.

Supernovae

The plural form of supernova.

Type II Supernova

One of the two types of supernova.

Stellar Clusters

Groups of stars that are gravitationally bound.

Open Clusters

Stellar clusters mostly found in the disk of the Milky Way.

Globular Clusters

Stellar clusters located mostly outside the disk of the Milky Way.

Luminosity

The amount of power a star radiates, measured in watts.

Apparent brightness

The amount of starlight that reaches Earth, measured in energy per second per square meter.

Brightness formula

Brightness = Luminosity / (4π (distance)²)

Luminosity formula

Luminosity = 4π (distance)² x (Brightness)

Parallax

The apparent shift in position of a nearby object against a background of more distant objects.

Parallax angle

The angle that depends on the distance of a star, changing as Earth orbits the Sun.

Alpha Centauri

A star that has about the same luminosity as the Sun.

The Sun

A star that appears brighter than Alpha Centauri due to its proximity.

Distance effect on brightness

If Alpha Centauri were three times farther away, its apparent brightness would be only 1/9 as bright.

M45 (Pleiades)

A specific open cluster in formation.

Parallax angle

p = parallax angle

Distance in parsecs

d (in parsecs) = 1 / p (in arcseconds)

Distance in light

years

Apparent brightness

how much energy you receive on Earth

Luminosity of most luminous stars

106 LSun

Luminosity of least luminous stars

10

Magnitude scale

m = apparent magnitude, M = absolute magnitude

Brightness comparison

apparent brightness of Star 1 / apparent brightness of Star 2 = (1001/5)^(m1−m2)

Luminosity comparison

luminosity of Star 1 / luminosity of Star 2 = (1001/5)^(M1−M2)

1st magnitude star

A 1st magnitude star is 100 times brighter than a 6th magnitude star.

Thermal radiation

Nearly all objects emit thermal radiation, including stars, planets, you.

Properties of thermal radiation

Hotter objects emit more light at all frequencies per unit area, emit photons with a higher average energy, and are bluer (whiter).

Surface temperature of hottest stars

50,000 K

Surface temperature of coolest stars

3,000 K

Phase of matter

Phase of matter depends on temperature.

Ionized gas

Plasma

Neutral gas

Gas that is not ionized.

Spectral line emission and absorption

Depends on the phase of matter.

Spectral types

Lines in a star's spectrum correspond to a spectral type that reveals its temperature.

Pioneers of stellar classification

Annie Jump Cannon and her collaborators at Harvard laid the foundation of modern stellar classification.

Most massive stars

? MSun

Binary star systems

About half of all stars are in binary systems.

Visual binary

In visual binaries, we can directly observe the orbital motions of these stars.

Eclipsing binary

In this case, we cannot observe the orbits because the two stars are so close to each other.

Spectroscopic Binary

A binary star system where the orbit cannot be seen, but can be determined by measuring Doppler shifts.

Mass Measurement

The process of determining the mass of stars using gravitational effects.

Direct Mass Measurements

Possible only for stars in binary star systems.

Kepler's Third Law

p² = (4π² / G(M1 + M2)) * a³, where p is the period, a is the average separation, and G is the gravitational constant.

Observables for Mass Measurement

Two out of three observables are needed: Orbital Period (p), Orbital Separation (a or r), and Orbital Velocity (v).

Orbital Velocity Formula

For circular orbits, v = 2πr / p.

Most Massive Stars

Can have a mass up to 100 MSun.

Least Massive Stars

Can have a mass as low as 0.08 MSun.

Supermassive Black Hole

Located at the center of the Milky Way, it has a mass of 4 million times that of our Sun.

Hertzsprung

Russell Diagram

Types of Stars in H

R Diagram

Main Sequence Stars

Stars that fuse hydrogen into helium in their cores, like the Sun.

Characteristics of Giants and Supergiants

They have lower temperatures, are redder, and must be larger in size to be bright.

Characteristics of White Dwarfs

They are bluer, have higher temperatures, and must be smaller in size to be faint.

H

R Diagram Depicts

Spectral Types

Classified as O, B, A, F, G, K, M based on temperature and color.

Significance of the Main Sequence

Main

Mass of Main

Sequence Stars

Luminosity and Temperature Relationship

Luminous main

Mass Measurements of Stars

Determine a star's luminosity and spectral type based on its mass.

Low

mass stars

Gravitational Equilibrium

The balance between gravity and thermal pressure in a star.

Luminosity

The total amount of energy emitted by a star per unit time, measured in terms of brightness and distance.

Temperature

The measure of a star's heat, determined from its color and spectral type.

Mass

The amount of matter in a star, determined from the period (p) and average separation (a) of binary

Massive star

A star that has a larger mass and shorter lifetime compared to less massive stars.

Life expectancy of 10 MSun star

Approximately 10 million years, due to using fuel 10,000 times faster than the Sun.

Life expectancy of 0.1 MSun star

Approximately 100 billion years, using fuel 0.01 times as fast as the Sun.

High Mass Main

Sequence Star

Low Mass Main

Sequence Star

Giants and Supergiants

Stars that have finished fusing hydrogen to helium and have expanded and cooled.

White Dwarf

The remnant core of a star that has shed its outer layers, resulting in a small, dense object.

Planetary Nebula

An expanding envelope of a star that escapes its gravitational field.

Open Cluster

A group of a few thousand loosely packed stars that formed almost at the same time.

Globular Cluster

A dense ball of up to a million or more stars bound together by gravity, all formed at the same time.

Main

sequence turnoff

Massive blue stars

Stars that have short lifetimes and die first in a star cluster.

Age of the Universe

Approximately 14 billion years.

Oldest Globular Clusters

Globular clusters that are about 13 billion years old, indicating they formed very early in the Universe.

Stellar Evolution Models

Comparative models used to determine the ages of star clusters based on their stellar properties.

Main

sequence stars

Core Hydrogen Exhaustion

The process when a star uses up about 10% of its total hydrogen fuel.

Mass & Lifetime Relationship

Less massive stars exhaust their fuel for nuclear fusion later than more massive stars.