The giant galaxy Messier 87 has finally been sized up, but outer parts that should have been there are missing, scientists now find. The galaxy is smaller than expected.

Messier 87 belongs to the Virgo Cluster of galaxies, the nearest galaxy cluster to our own Milky Way.

The relatively young cluster is located about 50 million light-years away in the constellation Virgo and contains many hundreds of galaxies.

A team of astronomers used the super-efficient FLAMES spectrograph at the European Southern Observatory's (ESO) Very Large Telescope in Chile to make ultra-precise measurements of a host of planetary nebulae in the outskirts of Messier 87.

(Planetary nebulae are the final phase in the life of sun-like stars, when stars eject their outer layers into space.)

Observing the light of the planetary nebulae in the Virgo Cluster from Earth is akin to looking at a 30-Watt light bulb from a distance of about 3.7 million miles (6 million kilometers), or about 15 times the distance between the Earth and the moon.

"It is a little bit like looking for a needle in a haystack, but in the dark," said team member Magda Arnaboldi of the ESO.

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The new observations, detailed in an upcoming issue of the journal Astronomy and Astrophysics, show that Messier 87's halo of stars is missing some members. The team estimates that it has a diameter of about a million light-years, which is significantly smaller than expected (though still three times the extent of the Milky Way's halo).

"This is an unexpected result," said team member Ortwin Gerhard of the Max-Planck Institute for Extraterrestrial Physics in Germany. "Numerical models predict that the halo around Messier 87 should be several times larger than our observations have revealed. Clearly, something must have cut the halo off early on."

Just what that something is, scientists aren't yet sure, though they have a few ideas.

The collapse of dark matter nearby in the galaxy cluster could account for the "cut-off." Another possibility is that another galaxy in the cluster, Messier 84, came much closer to Messier 87 in the past (about a billion years ago) and dramatically perturbed it.

"At this stage, we can't confirm any of these scenarios," Arnaboldi said. "We will need observations of many more planetary nebulae around Messier 87."

One thing the observations showed for sure though is that Messier 87 is on a collision course with another nearby neighbor, Messier 86.

"We may be observing them in the phase just before the first close pass," Gerhard said. "The Virgo Cluster is still a very dynamic place and many things will continue to shape its galaxies over the next billion years."

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