﻿ Oort constants

# Oort constants

The Oort rotation formulas for the differential rotation of the star system of the Milky Way were developed by the Dutch astronomer Jan Hendrik Oort ( 1900-1992 ).

1927 succeeded Oort proof of the rotation of our galaxy. Using the stellar he looked at the stars in the solar neighborhood, describing the differential rotation of the spiral arms. The main object of study was the spatial distribution of radial velocities and proper motions.

As the stars do not exactly follow the differential rotation of the Milky Way, but have additional peculiar velocities, the Oort rotation formulas do not apply to each individual star, but only on average over many stars ( Figure 2).

## Formulation

The Oort rotation formulas are:

With the Oort constants (current values, determined from the results of Hipparcos )

As well as for the galactic longitude of the star and its distance from the sun.

## Interpretation

Radial velocity and proper motion describe about the 360 degrees of the galactic longitude each have a double shaft with two maxima and minima ( Figure 2).

### A B

That is, the rotation curve of the Milky Way is close to the sun almost flat (slightly decreasing).

### A - B

Is the angular velocity of the rotation of the Sun around the center of the Milky Way.

This corresponds to an orbital period of the Sun around the center of the Milky Way (ie 230 million ) years, also called galactic year.

With the distance of the sun from the center of the Milky Way, this results for the solar orbital velocity, which agrees reasonably well with other observational data.

The other way around also be determined from the distance of the sun from the center of the Milky Way. For this, the speed of the sun must be known relative to objects that do not follow the rotation of the Milky Way (eg, globular clusters ).

• Milky Way
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