NASA Astronomy Picture of the Day

Mar 29, 2024

Galileo's Europa

Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, with improved calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. The Juno spacecraft currently in Jovian orbit has also made repeated flybys of the water world, returning images along with data exploring Europa's habitability. This October will see the launch of the NASA's Europa Clipper on a voyage of exploration. The spacecraft will make nearly 50 flybys, approaching to within 25 kilometers of Europa's icy surface.

https://apod.nasa.gov/apod/astropix.html?

Hubble Finds a Field of Stars

MAR 29, 2024

This image from the NASA/ESA Hubble Space Telescope shows a globular cluster called NGC 1651. Like another recent globular cluster image, NGC 1651 is about 162,000 light-years away in the largest and brightest of the Milky Way’s satellite galaxies, the Large Magellanic Cloud (LMC). One notable feature of this image: the roughly 120-light-year diameter globular cluster nearly fills the entire frame. In contrast, other Hubble images feature entire galaxies – which can be tens or hundreds of millions of light-years in diameter – that also more or less fill the whole image.

A common misconception is that Hubble and other large telescopes observe wildly differently sized celestial objects by zooming in on them, as one would with a specialized camera here on Earth. While small telescopes might have the option to zoom in and out to a certain extent, large telescopes do not. Each telescope’s instrument has a fixed ‘field of view’ (the size of the region of sky that it can observe in a single observation). For example, the ultraviolet/visible light channel of Hubble’s Wide Field Camera 3 (WFC3), the channel and instrument that collected the data used in this image, has a field of view roughly one twelfth the diameter of the Moon as seen from Earth. When WFC3 makes an observation, its field of view is the size of the region of sky that it can observe.

The reason that Hubble can observe objects of such wildly different sizes is two-fold. First, the distance to an object will determine how big it appears from Earth, so entire galaxies that are relatively far away might take up the same amount of space in the sky as a globular cluster like NGC 1651 that is relatively close by. In fact, there's a distant spiral galaxy lurking in this image, directly left of the cluster – though undoubtedly much larger than this star cluster, it appears small enough here to blend in with foreground stars! Secondly, images processors can stitch together multiple images spanning different parts of the sky into a mosaic to create a single image of objects that are too big for Hubble’s field of view.

https://science.nasa.gov/missions/hubble/hubble-finds-a-field-of-stars/