Gomez's Hamburger (IRAS 18059-3211): A Dying Star Behind a Dusty Disk

What looks like a burger in this Hubble picture is a star near the end of its life, seen through its own debris. The two bright buns are starlight bouncing off dust. The dark patty between them is the shadow of a thick disk of dust that circles the star and is turned exactly edge-on to Earth, so the star itself never shows.

Gomez's Hamburger lies very roughly 6,500 light-years away in Sagittarius. It carries the catalog name IRAS 18059-3211 and is classed as a proto-planetary nebula, the short-lived phase between a bloated red giant and the glowing shell of a true planetary nebula.

Why the star is hidden

The central star has a surface temperature of about 10,000 degrees Celsius (18,000 degrees Fahrenheit), hotter than the Sun but not yet hot enough to make its surroundings glow. Its light cannot get out through the plane of the disk. It escapes only above and below it, where it reaches dust that the star shed earlier and scatters off the grains toward Earth.

Nobody is sure why the disk is so thick. One possibility is that the central object is two stars, not one. If the star that threw off the nebula has a close companion and is spinning rapidly, it would shed material mostly from its equator, building up exactly this kind of dense belt.

A nebula before it lights up

Stars of roughly the Sun's mass end their lives in two steps. First the star swells into a red giant about 100 times its original diameter and pushes its outer layers into space. Then the exposed core, small and extremely hot, floods the expelled gas with ultraviolet light and makes it glow. That glowing gas is a planetary nebula, the kind of object seen in Hubble's views of the Ring Nebula and the Cat's Eye Nebula.

Gomez's Hamburger sits just before the second step. The remnant star is still relatively cool, so it gives off ordinary visible light and very little ultraviolet. The gas around it stays dark. What makes the nebula visible is dust mixed into the ejected material: microscopic grains that reflect the starlight the way fog makes a halo around a streetlight.

This phase does not last. Within less than a thousand years the central star is expected to grow hot enough to evaporate the dust, uncovering the star itself. Its ultraviolet light will then switch on the gas, and Gomez's Hamburger will become a glowing planetary nebula.

Discovery and the Hubble image

The object takes its name from Arturo Gomez, an astronomer at the Cerro Tololo Inter-American Observatory in Chile, who found it on sky photographs. Those photographs hinted at a dark band across the middle, but turbulence in Earth's atmosphere blurred the details too much to show what the band really was.

The Hubble image, taken on February 22, 2002 with the Wide Field and Planetary Camera 2, settled the shape. It combines 32 minutes of exposure through three broadband filters, F450W, F555W and F675W, which roughly match blue, green and red light, and it was released on August 1, 2002 as part of the Hubble Heritage series. At Hubble's resolution the band is sharp-edged and clearly a disk in silhouette, and the two lobes resolve into smooth fans of reflected light. The page Are the Colors Real? explains how three grayscale exposures like these are combined into one color picture.

Young or old? A look-alike problem

Edge-on dusty disks with light leaking out above and below are not unique to dying stars. Young stars that have only just formed show the same layout, because they are also surrounded by flattened disks of gas and dust, the raw material from which planets may form.

HH 30 in Taurus is the standard example. Its central star is hidden behind a thin, dark disk about 64 billion kilometers across, and the star's light reflects off the disk's upper and lower surfaces to make two reddish nebulae. Unlike Gomez's Hamburger, HH 30 also fires narrow jets of gas, shown green in this image, because the young star is still feeding from its disk. The two objects look alike for the same geometric reason, yet one sits at the start of a star's life and the other near its end.

Questions readers ask

Why is it called Gomez's Hamburger?

The name joins its discoverer and its shape. Arturo Gomez of the Cerro Tololo Inter-American Observatory found the object on sky photographs. In Hubble's image the two bright lobes of reflected light look like the halves of a bun, and the dark band of dust between them looks like the patty.

What is a proto-planetary nebula?

It is the brief stage after a Sun-like star has thrown off its outer layers but before its remaining core is hot enough to make that gas glow. The nebula is seen only by starlight reflected from dust. In Gomez's Hamburger this stage should end within less than a thousand years.

Is Gomez's Hamburger related to planets?

No. The phrase planetary nebula is historical: in small telescopes many of these nebulae look like round disks, similar to planets. They are shells of gas from dying stars. The disk around Gomez's Hamburger is debris from an old star, unlike the planet-forming disks around young stars such as HH 30.

How far away is Gomez's Hamburger?

The distance is uncertain. The best estimate published with the Hubble image was very roughly 2,000 parsecs, about 6,500 light-years, in the direction of the constellation Sagittarius.