M100 HST

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This comparison image of the core of the galaxy M100 shows the

dramatic improvement in Hubble Space Telescope’s view of the

universe. The new image was taken with the second generation

Wide Field and Planetary Camera (WFPC-2) which was installed

during the STS-61 Hubble Servicing Mission. The picture

beautifully demonstrates that the corrective optics incorporated

within the WFPC-2 compensate fully for optical aberration in

Hubble’s primary mirror. The new camera will allow Hubble to

probe the universe with unprecedented clarity and sensitivity,

and to fulfill many of the most important scientific objectives

for which the telescope was originally built.

Compare these images to those of the same region from KPNO

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An image of the grand design spiral galaxy M100 obtained with the

second generation Wide Field and Planetary Camera.

The WFPC-2 will allow the Hubble Space Telescope to be

used to attack one of the most fundamental questions in science:

the age and scale of the universe. Astronomers have many

“yardsticks” for measuring the scale of the universe, but lack a

good knowledge of how long these yardsticks really are. M100 is a

member of the Virgo Cluster of galaxies. By allowing astronomers

to resolve and measure individual stars in the Virgo Cluster — in

particular a special type of star called Cepheid variables, which

have well known absolute brightnesses — HST observations are

expected to provide a crucial measurement of this much needed

scale. (Only Space Telescope can make these types of observations.

Cepheids are too faint and the resolution too poor, as seen from

ground-based telescopes, to separate the images in such a crowded

region of a distant galaxy.)

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This sequence of pictures shows successive steps in optical

improvement from ground based telescopes to the newly improved

Hubble Space Telescope and demonstrates the unique capability of

the repaired HST. HST offers superb resolution, which allows

astronomers to distinguish individual stars in other galaxies.

The resolution also allows very faint stars to be seen. This set

of pictures demonstrates that the repaired HST can see stars

which could never before be detected.

Upper Left:

The new WFPC-2 is able to resolve individual bright stars in this

Virgo Cluster galaxy, which are indicated by arrows.

At this early image, it is not sure if the marked stars are bright Cepheid

variables, from which one could derive a distance. Over the following months,

however, astronomers were successful in identifying over 20 of these variable

stars with the refurbished HST, and determined this galaxy’s distance to be

56+/-6 million light years.

Upper right:

An exposure of the same field in M100, but taken with the old WPFC-1 and the

less sharp, uncorrected pre-refurbishment HST optics, on Nov 23, 1993.

The arrowed stars are no more visible. The brighter spots are probably

clusters.

Lower left:

Same view, computer enhanced. Still the bright individual stars stay hidden

in the diffuse background.

Lower right:

An image from the Earth-bound 200-inch telescope on Mt. Palomar, taken under

conditions slightly better than average. Even the rawer details visible with

the WPFC-1 remain obscured in a diffuse nebulous background.

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