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CosmosUp | July 9, 2020

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The Binary System SS Cygni Much Closer Than We Thought

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The Binary System SS Cygni Much Closer Than We Thought

The binary system SS Cygni contains a white dwarf and a red dwarf that complete orbits around each other in just 6.6 hours. This pair, which astronomers thought was 520 light-years away, actually is just 370 light-years from Earth, according to results published May 24 in Science.

SS Cygni is a dwarf nova, meaning that the white dwarf pulls matter from its companion star. This material forms a swirling disk that gravity funnels toward the white dwarf, causing an outburst every 49 days.

In other dwarf nova systems, when the disk deposits matter at a low rate, it becomes unstable, and that instability is what causes the bursts. However, when astronomers believed SS Cygni to be 520 light-years away, its periodic brightening did not fit within that model.

SS Cygni text

However, by combining data from 10 radio antennas into a 5,351-mile-wide (8,612 kilometers) telescope called the Very Long Baseline Array, astronomers were able to determine that SS Cygni is 150 light-years closer than they thought, meaning that it is intrinsically less bright and, thus, more unstable.

The revised distance means theories of dwarf nova outbursts work with this system, so instead of revising the theory, astronomers only have to revise their tape measures.

Innteresting Facts

Hubble catches galactic merger in action

2MASX J05210136- 2521450 is an ultraluminous infrared galaxy undergoing rapid star formation. Two smaller galaxies merged, their gas and dust colliding and collapsing into new suns, causing the structure to glow most brightly in wavelengths slightly longer than visible light’s. The collision also produced the galaxy’s strange shape: a bright center; intersecting, asymmetrical arms; and a “tidal tail”of material gravitationally torn from the original members. This image, released May 6, combines Hubble data in visible and near-infrared light.

black hole

Universal Densities

Objects in space sometimes have densities relatively similar to more familiar earthly ones, like the Sun’s core compared to gold. The densities listed are in kilograms per meter cubed:

Universal Densities


  1. Glen

    Under “Universal Densities”, Lab vacuum does not have a density of 1 kg/m^3. Something is messed up with this story.

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