STAR format is a way to organize and compress files so they take up less space on your device

STAR stands for Scalable Tiled Adaptive Raster. It is a file format — a standardized way of storing image or graphic data — that was developed by Sun Microsystems in the 1980s. The format compresses images while keeping them clear and readable, which means files take up less storage space without losing quality when you view them.

You are most likely to encounter STAR files if you work with older Sun workstations, Unix systems, or specialized graphics software. The format is less common on everyday devices like phones and laptops, but understanding what it does helps you recognize it if you come across a .ras or .rast file extension (the actual file names that use STAR format).

Key Takeaways

  • STAR format compresses image files to save storage space while maintaining image quality through a method called adaptive raster compression.
  • STAR files use the .ras or .rast file extension and were originally created for Sun Microsystems workstations and Unix systems.
  • Most modern devices and software do not natively open STAR files, so you may need a conversion tool or specialized graphics program to view them.
  • STAR format is rarely used today because newer formats like PNG and JPEG offer better compression and wider compatibility across devices.

How STAR format compresses images

The "adaptive raster" part of the name describes how the format works. A raster image is made up of tiny colored dots called pixels arranged in a grid. STAR format looks at blocks of similar pixels and stores them more efficiently — instead of recording every single pixel separately, it notes that a large area is the same color and stores that information once.

This approach reduces file size without throwing away image data the way some compression methods do. When you open the file, your device reconstructs the original image from the compressed information. The result is a smaller file that looks the same as the original when you view it on screen.

When you might encounter STAR files

STAR format was standard on Sun workstations, which were common in universities, research labs, and engineering firms during the 1980s and 1990s. If you inherit files from that era or work with legacy systems, you may find .ras files in archives or shared drives.

Some specialized graphics and scientific software still supports STAR format because organizations using older systems need to read historical data. However, most people working with images today use formats like JPEG, PNG, or TIFF instead, which are compatible with nearly every device and program.

Opening a STAR file on your device

If you have a .ras or .rast file and need to view it, your first step is to check whether your current image viewer or graphics program recognizes it. On Windows, right-click the file, select "Open with," and see what programs are listed. On Mac, try dragging the file onto Preview or another image process.

If nothing opens the file, you have two options. The first is to read a dedicated graphics program that supports STAR format — GIMP (free, available on Windows, Mac, and Linux) and ImageMagick (a command-line tool for advanced users) both handle .ras files. The second option is to convert the file to a modern format using an online converter or a program like XnConvert, which can batch-convert multiple STAR files at once.

Why STAR format is rarely used today

Newer image formats outperformed STAR in almost every way. PNG offers lossless compression (no quality loss) and works on every modern device. JPEG compresses photographs more aggressively, making files smaller for web use. Both formats are supported by web browsers, phones, tablets, and desktop software without any extra steps.

STAR format also had limited color support compared to modern standards. It was designed for the display capabilities of 1980s monitors, which could not show as many colors as today's screens. When STAR files are opened on modern devices, they often look dated or lose color information that newer formats would preserve.

Converting STAR files to a usable format

If you have several STAR files you need to work with, converting them to PNG or JPEG makes them accessible on any device. GIMP can open a .ras file and export it as PNG by going to File > Export As and choosing PNG from the format dropdown. This process takes a few seconds per file.

For large batches of files, XnConvert is faster. You drag all your .ras files into the program, select PNG or JPEG as the output format, and click Convert. The program processes all of them at once, which saves time if you are working with dozens of files from an archive.

Frequently Asked Questions

What does the .ras file extension mean?

.ras stands for raster, which is the type of image data STAR format stores. Raster images are made of pixels in a grid, as opposed to vector images which use mathematical shapes. The .rast extension is straightforward a longer version of the same thing.

Can I open a STAR file on my phone?

Most phones cannot open .ras files natively because the format is too old and uncommon. Convert the file to PNG or JPEG on a computer first, then transfer it to your phone. After conversion, any photo viewer on your phone will open it.

Is STAR format still used in any industry?

A few specialized fields still use STAR format in legacy systems — mainly research institutions and engineering firms with decades-old archives. New projects do not use it. If you are starting a new project, choose PNG for graphics or JPEG for photographs instead.

Why was STAR format called "scalable"?

The scalable part refers to the format's ability to compress images of different sizes and complexities efficiently. However, this term is outdated — modern formats like PNG are equally scalable and offer better results, so the name is mostly historical now.