## Astronomy

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# Rotation Curve

Rotation Curve
Galactic rotation curves plot a galaxy's circular velocity (which can be measured using the of of the trailing and leading sides as viewed from the Earth) vs. the distance from the center of rotation.

Rotation curves: As discussed in the disk-dynamics section, real disk galaxies show flat rotation curves which require a density distribution approximately scaling as 1/R, so that the mass-to-light ratio M/L rises rapidly into the outer parts of these galaxies.

ROTATION CURVE - Plot showing how orbital velocity (V) varies with distance from the centre of an object (R).

Rotation Curve
A quantitative description of how fast each part of a galaxy is rotating about the center. A rotation velocity is the velocity of a rotating galaxy at a certain distance from the center of the galaxy.

Rotation Curve
A graph of orbital velocity versus radius in the disk of a galaxy.
RR Lyrae Variable ...

~ - A plot of the speed of revolution of the stars and gas in a galaxy versus distance from the center of the galaxy
RR Lyrae Star - A member of a class of giant pulsating stars, all of which have pulsation periods of about 1 day ...

The ~ is a plot of the orbital velocity of the clouds around the galactic center vs. their distance from the Galaxy center. The term ``rotation'' in this context refers to the motion of the galactic disk as a whole---the disk made of stars and gas clouds appears to spin.

The Galactic ~ plots the orbital speed of matter in the disk versus distance from the Galactic center. By applying Newton's laws of motion, astronomers can determine the mass of the Galaxy.

~s for 3 Spiral Galaxies - Galaxy Image(left), Spectrum (center - photographic negative), & Plot (right).
The flatness of the ~ with no downward turn indicates that the mass distribution extends far beyond ...

~ :
rapid rise, then constant (at ~200 km/s)
differential rotation (inner stars overtake)
dark matter extends beyond stars (unknown) ...

~ Plot of the orbital speed of disk material in a galaxy against its distance from the galactic center. Analysis of ~s of spiral galaxies indicates the existence of dark matter.

~ of a typical spiral galaxy: predicted (A) and observed (B). The distance is from the galactic core.

A ~ is a plot of speed versus distance from the center of an astronomical system.

Galactic ~s, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this.

Since then the ~s of many more galaxies were studied (bigger telescopes, much more sensitive electronic light detectors) and flat ~s were the general rule.

~s as evidence of a dark matter halo
The presence of dark matter in the halo is demonstrated by its gravitational effect on a spiral galaxy's ~.

Figure 16. The ~ for the Milky Way. The curve varies at different distances from the center. Our location is at roughly 8 kpc from the center. Graph is from a paper by Clemens, 1985.

1977 - Brent Tully and Richard Fisher discover the Tully-Fisher relation between the luminosity of an isolated spiral galaxy and the velocity of the flat part of its ~,
1978 - Steve Gregory and Laird Thompson describe the Coma supercluster,
1978 - Vera Rubin, Kent Ford, N.

A plot of orbital speed versus the distance from the galactic center is known as a ~.
The ~ along the above formula points to mystery regarding the Galaxy.

Because galaxies are so faint, measuring the ~ is a difficult business. But by the early 1970's, instruments advanced far enough to allow very precise measurements of galaxy rotation from the center out to the visible edge of the disk.

Among this data was each galaxy's '~,' a graph that plots the rotational speed of the stars in the galaxy as a function of their distance from the galaxy's center. These curves were successfully fit to curves produced using the new theory.

For example, the quantity and distribution of luminous matter within disk galaxies cannot account for the ~s observed, implying a significant invisible component.

The gravitational potential of the bulge is important for the dynamical behavior of the inner regions of the galaxy, but further out, the mass of the disk and the dark matter dominate, producing flat galactic ~s.

Measuring the Doppler shifts of the great interstellar clouds of hydrogen that line the Milky Way's spiral arms allows astronomers to measure the ~ of the Galaxy and infer its mass, or do the same for any of the myriad other galaxies our telescopes can see.

and Ho = 500 km/sec/Mpc to boot!
Symmetric Theory: D - an unclassifiable mess. Example: "a flat ~ implies solid body rotation."
Why the Big Bang is Wrong: F - more tired light but with the added failure of a mechanism that just won't work.

~
A graph of orbital velocity versus radius in the disk of a galaxy.
RR Lyrae Variable
Variable stars with periods of 12-24 hours, common in some globular clusters.
S