This is a composite image of the disk and jet from DL Tauri, a star that is still forming. The disk is comprised of dust and gas, and astronomers believe the Sun had a disk like this when it was young, about five billion years ago, and that the Earth and other solar system objects formed from material in its disk.

DL Tauri is relatively close, about 450 light-years from Earth in the direction of the constellation Taurus “The Bull”. (One light year is about six trillion miles.) The star is about 70 percent as massive as the Sun, and is currently a K-type star, somewhat cooler and dimmer than the Sun. DL Tauri is relatively young, about 2 million years old, a small fraction of its estimated lifespan of more than ten billion years.

The inset is an image from the Hubble Space Telescope, and the larger image is from the Astrophysical Research Consortium’s 3.5-meter telescope at the Apache Point Observatory, in Sunspot, N.M. These images are of interest to astronomers because they are trying to discover how stars and planets form. However, the birth process takes millions of years, so astronomers survey many young stars at various ages, hoping to capture them at different stages in their development to build a continuous history from the examples, much as an alien explorer might take pictures of children in different grades to quickly understand how humans grow up.

In the Hubble image, the disk surrounding DL Tauri is the circular reddish-orange area, and the gas jet is the fuzzy line extending toward the bottom left of the image. The disk extends about 42 billion miles (approximately 67 billion kilometers) from the star, more than ten times the farthest distance of Pluto from the Sun. The jet is about 81 billion miles (130 billion kilometers) long, and was discovered with this Hubble image. The irregular black area in the center of the picture is the coronagraph on Hubble, which blocks direct light from the star so the much fainter surroundings can be seen.

A small bulge on the top edge of the disk is actually another gas jet directed away from the Earth, with the disk blocking much of it from our view.

The Apache Point photo has a much wider field of view, and it was used to show more of the gas jets. The light, circular areas in the Apache Point image are part of the gas jet. These areas, called Herbig-Haro objects (HH objects), are regions in the jet where rapidly-moving gas slams into slower gas, causing the jet to glow from the shock of the collision.

DL Tauri’s parent cloud of gas and dust collapsed under its own gravity, causing the star to form. Since the material can’t fall onto the star all at once, it forms a disk around the star, like water swirling around a drain. The disk flares out at the edges due to the competing influences of the star’s radiation and gravity. The star’s radiation heats the disk, causing it to expand, but the star’s gravity, which is stronger close to the star, flattens the part of the disk near the star. If a disk is directed toward the Earth, like the one surrounding DL Tauri, the flared outer edges will give it a cup-shaped appearance. The system will act like a flashlight, with the star as the bulb and the cup-shaped disk acting like a flashlight’s parabolic mirror as it reflects the star’s light toward Earth. If the disk were flat, it would only be bright in the center near the star, because intervening matter would block starlight from the edges, and it would appear much smaller in the Hubble images, if it could be seen at all.

Dust particles reflect light most strongly if they are about the same size as the wavelength of the light. The light used in these photos includes the full range of visible light, extending in the Hubble image into the ultraviolet and the infrared, both of which are invisible to the human eye. Based on the wavelengths of this light, astronomers deduce that the dust grains in the disk are about 1 micron across, or about the size of smoke particles.

Note the faint darker bands in the disk on the bottom left side. Astronomers aren’t sure if these are regions where planets may be forming and clearing lanes in the disk with their gravity, or if the bands are just shadows, either from irregularities in the disk or dust clouds between the star and the disk.

If a young star has jets, not all of the matter falling on the star becomes part of the star. Between one and ten percent of the infalling matter is ejected in jets from the poles of the star, by a process that is not fully understood. The gas that comprises these jets typically moves at high speeds, about 125 to 250 miles per second (approximately 200 to 400 km/sec). Astronomers can estimate a jet’s age from its size if its speed is known. If further observations reveal that the jet in DL Tauri is moving at typical speeds, astronomers estimate DL Tauri has been ejecting gas jets for at least a few centuries, based on the size of the jets in the Apache Point photo.

This image is available on the internet in a high-resolution format suitable for printing:
http://www.gsfc.nasa.gov/topstory/20020606dltauri.html

IMAGE CREDIT: NASA/Apache Point Observatory/Carol Grady