Supermassive Black Holes

Preserving a Galactic Treasure

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Parts of a Spiral Galaxy

(Supermassive Black Hole)

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Supermassive Black Holes (SBH)

• Sgr A* (pronounced: Sagittarius A-star)– Strong radio source in

the center of the MilkyWay galaxy (our galaxy)

– Mass = 4 x 106 M• M87 (supergiant

elliptical galaxy)– Contains a SBH at its

center– Mass = 6.6 x 109 M

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Proof of Existence for Sgr

A*

The mass of the central (and not visible) object can be found from the velocities of nearby stars orbiting it.

These images/animations were created by Prof. Andrea Ghez and her research team at UCLA and are from data

sets obtained with the W. M. Keck Telescopes.

How to Weigh a Black Hole

• Assume the mass of the star is insignificant compared to the mass of the central object– Assumption based on the fact that the central object

does not appear to move.– This eliminates the “M2” term.

• Use a form of Kepler’s Third Law.

See equation practice sheet for quantities and units.

𝑎3=(𝑀 1+𝑀 2)𝑃2

Schwarzschild Radius

• How big is a black hole? Based on the mass, we can calculate the distance from the singularity from which no light can escape.– This is called the Schwarzschild radius.

• Equation is derived using the escape velocity.

See equation practice sheet for quantities and units.

Diameter of a Black Hole

• If we have both the linear diameter (RSch) and the distance to the black hole, we can calculate the angular diameter.

• Relate these using the small angle formula

See equation practice sheet for quantities and units.

Trying to Observe the Galactic Center

• Radio – Observable but very low resolution• Microwave – Observable but requires VLBI for

good resolution• Infrared – Observable but highly blurred by

Earth’s atmosphere• Visible – Cannot pass through all the dust in the

galactic disk• UV/X-ray/Gamma – Not observable on Earth

What We Expect to see in Radio When We Observe Sgr A*

Used with Permission

Supermassive Black Hole Activity