The evolution of the star: Stars are born out of clouds of gas and dust. A star generates energy by converting hydrogen into helium deep in the centre (the core). As most of the hydrogen turns into helium in the core, the core runs out of “fuel”. The core contracts under pressure and becomes hotter and denser. It then begins to convert helium into carbon. As the core contracts and heats up, the heat causes the outer atmosphere of the star to expand greatly (Red Giant): energy increases and the core contracts.

Low mass star evolution: For a low-mass star (such as the sun), the core will never be hot and dense enough to further the nuclear process. When helium runs out in the core, the stars no longer generate energy. It slowly cools and its outer atmosphere blows away into the interstellar medium (planetary nebula). Only the core of the star, which is called a white dwarf, remains

High Mass Star Evolution: (>8 times the mass of the sun), the core-contraction, atmosphere-expansion process continues (supergiant). The core of the star slowly turns into heavier elements. When the core turns into iron, the nuclear process stops and the star no longer generate energy. The end of energy production causes the core to collapse abruptly into a very dense object (neutron star or black hole). Core collapse sends an enormous amount of energy outward, causing a huge explosion in the outer atmosphere (supernova explosion) – many heavy elements are produced during such an explosion

Composition: All stars are made of roughly 70% hydrogen, 28% helium and 2% “metal”

In the Hertzsprung-Russell (HR) Diagram, each star is represented by a dot. There are lots of stars out there, so there are lots of dots. The position of each dot on the diagram tells us two things about each star: its luminosity (or absolute magnitude) and its temperature.

The vertical axis represents the star’s luminosity or absolute magnitude. Luminosity is technically the amount of energy a star radiates in one second, but you can think of it as how bright or how dim the star appears. Depending upon the textbook you use, the labels on the HR diagram could be a little different. Luminosity is a common term, as is absolute magnitude. Absolute magnitude is the intrinsic brightness of a star. In either case, the scale is a “ratio scale” in which stars are compared to each other based upon a reference (our sun).


The horizontal axis represents the star’s surface temperature (not the star’s core temperature – we cannot see into the core of a star, only its surface)! Usually this is labeled using the Kelvin temperature scale.  But notice: In most graphs and diagrams, zero (or the smaller numbers) exist to the left on the diagram. This is not the case here. On this diagram, the higher (hotter) temperatures are on the left, and the lower (cooler) temperatures are on the right. Some HR diagrams include the color of stars as they can be seen through filters on spectrophotometers. This is also a “ratio scale.”

So how do you read the HR diagram? Well, let’s look at some basic regions on it. A star in the upper left corner of the diagram would be hot and bright. A star in the upper right corner of the diagram would be cool and bright. The Sun rests approximately in the middle of the diagram, and it is the star, which we use for comparison. A star in the lower left corner of the diagram would be hot and dim. A star in the lower right corner of the diagram would be cold and dim.

The Main Sequence (MS)

–The curved band diagonally across from upper left (hot and bright) to lower right (cold and faint)

–Most stars (90%) fall within this band

–There are more cool MS stars than hot ones

–The Sun is a MS star

–MS stars are also called Dwarfs

Why is it that most of the stars in the sky seem to be MS stars?

–During the lifetime of a star, its luminosity and surface temperature changes, taking the star through the MS stage > GB stage > WD stage.

–If the star is much more massive than the Sun, than it will go through the MS stage > GB > SG stage

–Since Hydrogen is the most abundant element in a star, the MS stage is where every star spend 90% of their lives

The Giant Branch (GB)

–Brighter than MS stars of the same temperature (e.g., Arcturus, Pollux)

•These stars must have larger radii than MS stars

Supergiant region (SG)

–Much more luminous than GB stars of the same temperature (e.g. Betelgeuse, Antares)

•They are larger (in radius) than GB stars

–Cool SG stars are called Red Super giants

–Hot SG stars are called Blue Super giants

–SG stars are very rare (< 1%)

White Dwarfs (WD)

–Very hot but have low luminosities (e.g., Sirius B)

–Very tiny (size of Earth)

The Inverse-Square Law for Radiation: The amount of radiation falling on an observer decreases as the distance between the source and the observer increases.

Stellar Luminosity and Brightness:

The total amount of energy emitted by a star in one second is called its luminosity. The brightness of a star usually refers to its apparent brightness, or how bright the star appears to the observer. If two stars have the same luminosity, but star A is further away from the observer than star B, then star B will appear brighter – the Inverse Square Law for Light.

Measuring Apparent Brightness:

The apparent brightness of a star may be measured by collecting light from the star–This technique is called Photometry. CCD cameras are used to collect visible light from stars; some CCDs are also sensitive to some parts of UV and infrared parts of the EM spectrum. We can also use detectors sensitive to other wavelengths (e.g., X-ray, Microwave, radio wave). By adding the intensities from all different parts of the EM spectrum, we can determine the total apparent brightness of a star

Luminosity, Apparent Brightness and Stellar Distance:

  • The apparent brightness of a star can always be determined through Photometry
  • If the distance of a star is also known, then the luminosity of the star can be calculated
  • Conversely, if the luminosity of a star is known, then the distance of the star can be calculated

Stellar Magnitude System: 1st magnitude stars roughly 100 times brighter than 6th magnitude stars.

More negative magnitude= brighter

The Apparent Brightness of a star is measured by its Apparent Magnitude

Parallax: the apparent shifting of position of an object (relative to more distant background objects) due to changing perspective.

Stellar Parallax: nearby stars appear to change positions in the sky (relative to the background stars) due to the orbit of the earth around the sun.

  • Only nearby stars (d < 500 ly) have parallax angles large enough to be measured and for more distant stars, (d > 500 ly), distances cannot be measured directly.

Measuring Surface Temperatures

  • In determining the surface temperature of a star is called photometry or  stellar spectroscopy
  • In photometry, the intensities of light passing through a set of filters are recorded and the blackbody spectrum of the star can be plotted

o   Wien’s Law is then used to determine the surface temperature of the star

•Star surface temperature > about 10,000 K

–Weak Balmer lines and weak or no metal lines

–Strong ionized helium lines

•Star surface temperature about 10,000 K

–Strong Balmer lines

–Moderate ionized metal lines

•Star surface temperature < about 10,000 K

–Weak Balmer lines

–Strong neutral (not ionized) metal lines

–Very cool stars have strong molecular lines

Star Categories or Types

  • Stars are grouped together based on their spectral features
  • The hottest (> 30,000 K) stars are O-type stars
  • A-type stars are about 10,000 K
  • The coolest (< 3500 K) stars are M-type stars (also the most numerous)
  • The classification scheme is: O, B, A, F, G, K, M
  • Subdivision also in use today:
  • –O5-O9, B0-B9, A0-A9, etc
  • –e.g., the Sun is a G2 stars

Luminosity (Higher-mass star has much higher luminosity)

  • The atmosphere of a large star (e.g., a giant or a SG) has low density and low pressure

–Its spectral lines are narrower

  • A small star (e.g., a dwarf) has high atmospheric density and pressure

–Its spectral lines are broader

  • Luminosity classes are ranking of stars’ radii

–Supergiants are classes Ia and Ib

–Classes II, III, IV are different types of giants

–Dwarfs are class V

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