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Astronomers can measure any given star's radial velocity by examining its spectrum-light spread out into its constituent wavelengths. If an object is moving toward us,its spectral lines shift to shorter wavelengths; if it's moving away, the lines swing to longer wavelengths. The higher the velocity, the greater the shift. Although this sort of spectral analysis is straightforward for nearby stars. it becomes far more difficult for distant stars in the Milky Ways outer halo. Even large telescopes can't gather enough of the light. For this reason, astronomer Ulrich Heber conjectures that there are probably several low-mass hypervelocity stars yet to be discovered. Although these diminutive objects live longer than B type stars, which are extremely luminous and blue,they radiate much less light.