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H-R Diagram

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H-R Diagram
Hertzsprung-Russell diagram. A diagram of stars arranged according to their luminosity (measured on the y axis) and temperature (on the x axis). In the early part of the twentieth century, the Danish astronomer E. Hertzsprung and the American astronomer H.

H-R Diagram: The H-R (Hertzsprung-Russell) diagram is used by astronomers to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. Most stars fall into the Main Sequence, which runs from upper left to lower right.

H-R diagram
Hertzsprung-Russell diagram; a plot of the luminosity (or absolute magnitude) of stars against their surface temperature (or spectral type).

H-R Diagram
A plot of the intrinsic brightness versus the surface temperature of the stars. It separates the effects of temperature and surface area versus spectral type, but also luminosity versus surface temperature or color.
Half-Life ...

H-R Diagram
The Color-Magnitude Diagram is a graph upon which stars are plotted by spectral type and actual luminosity. It is named for the two scientists Russell and Hertzsprung who first used it in 1913.

~ from Helium Burning to White Dwarf.
7. Helium Burning Main Sequence ...

~s can be used in a variety of stellar applications. A few of them are discussed below.
Classification ...

The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage.

Main Sequence & Nucleosynthesis
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The Life & Death of Stars
Astrophysics home ...

~: A graph that uses two stellar properties, such as luminosity versus surface temperature, as its axes. Individual stars are positioned on the graph according to their properties (Absolute Magnitude, and Spectral type correspond to luminosity and temperature).

[edit] Notes
^ a b c d e f g These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer while generally adding only to the M class.
^ This rises to 78.

The ~ is also called a color-magnitude diagram because the absolute magnitude is usually plotted vs. the color. The ~ below is for all stars visible to the naked eye (down to apparent magnitude = +5) plus all stars within 25 parsecs.

In an ~ for a cluster of stars, the point where the main sequence turns off toward the upper right. The main-sequence turn-off, showing which stars in the cluster have evolved to become red giants, is an indicator of the age of the cluster.
major axis - (n.) ...

Once the ~ was popularized, a new method of determining the distances to stars was found - that of spectroscopic parallax. Actually, this is a rather confusing term, since there is no parallax angle measured.

A composite ~. The magnitudes and temperatures (measured by photometry) of stars in several star clusters are plotted. The position of a group of points in the diagram can tell us how old that particular star cluster is.

Determined by ~s and stellar models; color and luminosity of stars leavingmain sequence is a measure of age.
albedo ...

Figure: An ~ showing the evolutionary track for a one solar massed star. Main features include (1) Main sequence, (2) Red Giant Branch (RGB), (3) Helium Flash (4) Horizontal branch, (5) Asymptotic Giant Branch (AGB), (6) Planetary nebula ejection, (7) White Dwarf.

The Hertzsprung-Russell diagram, or simply the ~, plots star color along the horizontal axis and luminosity along the vertical axis. To make the chart fit on a single page, we actually plot the log10 of the luminosity.

Highly Polarized Quasar
See Hertzsprung-Russell Diagram. [C95]
HRS ...

Large, cool, highly luminous stars in the upper right of the ~. Typically 10-100 times the diameter of the sun.
Glacial Period
Time when glaciers advance and engulf huge sheets of land
A sudden change in the period of a pulsar.
Globular Star Cluster ...

main sequence Major distributional segment of the stars running diagonally across the ~ from the upper left to the lower right.

When stars are burning hydrogen to helium in their cores, they fall on a single curve in the luminosity-temperature plot known as the ~ after its inventors, Hertzsprung and Russell. This track is known as the main sequence, since most stars are found there.

The ~ provides astronomers the stellar "big picture," a handful of conceptual hooks on which to hang billions of stars. The most prominent feature is the main sequence, where stars spend the majority of their luminous life.

As its name implies, the horizontal branch is a horizontal, linear feature seen in cluster ~s, meaning that stars on the horizontal branch all have similar luminosities, but can have a range of effective temperatures.The horizontal branch is only seen in low-metallicity clusters.

These are supergiants in the instability strip on the ~, undergoing regular pulsations that are expressed by luminosity and temperature variations. Their high optical luminosity makes them easy to pick out (though, being rather massive stars, they don't occur in elliptical galaxies).

Of spectral and luminosity type B8 -- previously A5-F0 -- IIpe (Grady et al, 1996, in pdf, page 176), this star is still contracting towards the main sequence (see ~ for pre-main sequence tracks for seven stars of different mass).

Ages for star clusters can be estimated by comparing the ~ for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated.

main-sequence turnoff Special point on an ~ for a cluster. If all the stars in a particular cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch.

A new star will fall at a specific point on the main sequence of the ~.

birth line: In the ~, the line above the main sequence where protostars first become visible.
BL Lac objects: Object that resembles a quasar; thought to be the highly luminous core of a distant galaxy emitting a jet almost directly toward Earth.

asymptotic giant branch Path on the ~ corresponding to the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time.

Asymptotic Giant Branch (AGB) - The portion of the ~ occupied by enormous, cool stars with helium-burning shells
Aten Asteroid - An asteroid having an orbit with semi-major axis smaller than 1 AU
Atom - A particle consisting of a nucleus and one or more surrounding electrons ...

A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (~), allows the current age and evolutionary state of a particular star to be determined.

Hertzsprung and Russel introduce ~ that shows how the characteristics of stars are related.
1914 A.D.

Hertzsprung-Russell Diagram (~)
A plot of luminosity versus temperature for a group of stars.

Since they occupy a diagonal swath across the L vs. T graph (AKA the Hertzsprung-Russell or ~), it's known as the "instability strip." Other variable stars (including the Mira variables and the RR Lyrae variables) fall along different parts of this strip.

In the prime of their lives, when stars burn hydrogen in their core, there's a clear and simple relationship between a star's color and brightness. Nearly a century ago, astronomers developed a way to illustrate this relationship with what's now called the ~, ...

An Evolutionary Track is the change in location of a star on the Hertzsprung - Russell (H-R) Diagram. As a star ages and evolves, you can trace out its history on the ~.

See also: See also: What is the meaning of Star, Astro, Sun, Astronomy, Earth?

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