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How Much Do Stars Weigh?

A planet/moon transit that may be imaged by NASA’s Kepler spacecraft.

A planet/moon transit that may be imaged by NASA’s Kepler spacecraft.

By David Reneke

Dave Reneke brings news from the space and astronomy communities around the world.

How do astronomers weigh a star that’s trillions of kilometres away and way too big to fit on a bathroom scale? In most cases they can’t, although they can “guesstimate” using computer models of stellar structure.

New work by Dr David Kipping of University College, London says that in special cases we can weigh a star directly. If the star has a planet, and that planet has a moon, and both of them cross in front of their star, then we can measure their sizes and orbits to learn about the star. This is cutting-edge astronomy.

Astronomers have found more than 90 planets that transit, or cross in front of, their parent stars. By measuring the amount of starlight that’s blocked they can calculate how big the planet is relative to the star, but they must first know the star’s actual size. Computer models are one thing but real measurements are best.

Basically, they measure the orbits of the planet around the star and the moon around the planet. Then, using Kepler’s Laws of Motion, it’s possible to calculate the mass of the star. Plugging those numbers into Kepler’s Third Law yields the density of the star and planet. Sounds easy? It is, if you get the maths right.

Finally, and for added confirmation, scientists measure the star’s wobble due to the planet’s gravitational tug, which is known as the “radial velocity”. By combining the measured velocity with the relative masses they can calculate the mass of the star directly.

Kipping hasn’t put his method into practice yet, since no star is known to have both a planet and moon that transit. If NASA’s planet-finding Kepler spacecraft should discover several such systems, and it more than likely will, Kipping will be ready to weigh them.

The Most Distant Galaxy Ever Measured
I remember being taught one thing very early in my career about the universe, and it’s always stuck with me. When you look at the stars you’re looking back in time. When you use a telescope you’re using a “time machine”. I’ve never forgotten it.

A European team of astronomers, using their biggest time machine – the European Southern Observatory’s Very Large Telescope (VLT) – has measured the distance to the most remote galaxy so far. By carefully analysing the very faint glow of the galaxy, they were actually seeing it when the universe was in its infancy – a mere 600 million years old. For the purists that amounts to a red shift of 8.6!

These are the first confirmed observations of a galaxy whose light is clearing the opaque hydrogen fog that filled the cosmos at this early time. In effect, astronomers are seeing “baby images” of the Universe, an incredible feat for a developing field of science that is still so relatively young.

According to Matt Lehnert of the Observatoire de Paris, astronomers using the VLT’s powerful light-analysing spectrograph have, they believe, accurately plotted the enormous distance factor. This dim light falls mostly in the infrared part of the spectrum because its wavelength has been stretched by the expansion of the universe – an effect known as red shift.

At this early time, less than a billion years after the Big Bang, the universe was not fully transparent. Electrons and protons combined to form primordial hydrogen gas that absorbed the fierce ultraviolet light from young galaxies. The period when this “fog’ was still being cleared by this ultraviolet light is known as the era of “reionisation”.

This cool dark gas was the main constituent of the universe during the so-called Dark Ages, when there were no luminous objects. This phase eventually ended when the first stars formed, planets came along – then eventually you and I.

The rest, as they say, is history!

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at