How Astronomers Use Webb & Hubble for Star Formation

This guide explains the complementary roles of the James Webb Space Telescope (JWST) and the Hubble Space Telescope in studying star-forming regions. You’ll learn the typical workflow astronomers follow, from target selection to data analysis, and the practical steps for combining observations from both observatories.

What You’ll Need

• Access to archival data from the Mikulski Archive for Space Telescopes (MAST) – free after a short registration.
• Basic familiarity with FITS file format and a data analysis tool like Python (with astropy, photutils) or DS9.
• An understanding of astronomical coordinate systems (RA/Dec) and common units (arcseconds, parsecs).
• Optional: a proposal for new observing time – requires a detailed science case and target list.

Step-by-Step

1. Define the science question. Decide what you want to learn: e.g., the initial mass function (IMF) of stars, the structure of protoplanetary disks, or the chemistry of the interstellar medium. This determines which filters and instruments you need.
2. Select a star-forming region. Choose a well-known target like the Orion Nebula, the Carina Nebula, or a more distant giant molecular cloud. Check that the region has both Hubble and Webb archival coverage – many iconic regions have been observed by both.
3. Download the data from MAST. Use the MAST portal or astroquery.mast to search by target name or coordinates. Filter for the specific instruments: Hubble’s WFC3/UVIS or ACS, and Webb’s NIRCam or MIRI. Download the calibrated (level 2 or 3) FITS files.
4. Align the images. Since Hubble and Webb have different pixel scales and orientations, you must reproject them to a common grid. Use astropy’s reproject package or the TweakReg task in DrizzlePac. Match stars visible in both images to compute the astrometric solution.
5. Create a color composite. Assign each filter to an RGB channel: typically short-wavelength (blue) to blue, mid-wavelength (green) to green, and long-wavelength (red) to red. For Webb, use NIRCam filters like F090W, F150W, and F200W; for Hubble, use F275W, F606W, and F814W. Adjust the stretch (e.g., logarithmic or asinh) to bring out faint features.
6. Identify young stellar objects (YSOs). Use photometry to measure fluxes in multiple bands. YSOs often show infrared excess – brighter in longer wavelengths than expected from a bare photosphere. Plot a color-color diagram (e.g., H-K vs. K-[4.5]) to classify Class 0, I, II, and III protostars.
7. Measure the dust and gas structure. Use Hubble’s sharp optical images to trace ionized gas (H-alpha emission) and reflection nebulae. Use Webb’s infrared images to see through the dust, revealing embedded protostars and outflows. Compare the two to map the evolutionary sequence from collapsing core to pre-main-sequence star.
8. Analyze the data quantitatively. Run source extraction (e.g., using photutils or SExtractor) to build a catalog of sources. Compute the luminosity and temperature of each source by fitting spectral energy distributions (SEDs) – combine Hubble’s shorter wavelengths with Webb’s longer ones to cover a wide range.
9. Compare and interpret. Overlay the catalogs from both telescopes. Look for objects visible in Webb but not Hubble – these are likely deeply embedded. Look for objects visible in Hubble but faint in Webb – these may be older, dust-free stars. This differential visibility is key to understanding the timescale of star formation.
10. Publish and share. Create figures with clear annotations, release the data products (e.g., aligned mosaics, source catalogs) to a public repository, and write up your findings for a journal like the Astrophysical Journal.

Common Mistakes

• Ignoring astrometric offsets: Hubble and Webb images are not automatically aligned; even a small offset (a few arcseconds) can ruin a color composite or source matching. Always run an astrometric correction.
• Using different pixel scales without resampling: Hubble’s WFC3 has a pixel scale of ~0.04 arcsec/pixel, while Webb’s NIRCam is ~0.031 arcsec/pixel. If you don’t reproject, features will be misaligned.
• Mixing units: Fluxes may be in different units (e.g., MJy/sr vs. electrons/s). Convert to a common unit (e.g., Jy) before combining.
• Assuming all infrared sources are YSOs: Background galaxies and evolved stars can mimic infrared excess. Use color criteria and check the morphology (YSOs are often point-like or have nebulosity).
• Forgetting to subtract background: Star-forming regions have bright nebulosity; without background subtraction, photometry will be biased.

Tips

• Start with a small, well-studied region like the Orion Nebula Cluster – the data are abundant and the science is well-calibrated.
• Use the JWST/HST archive “pipeline” products – they are already calibrated and easier to work with than raw frames.
• When creating color images, use a non-linear stretch (e.g., asinh) to show both bright and faint features.
• For photometry, use the same aperture size in both images (after resampling) to ensure consistent measurements.
• Check the MAST “Hubble Legacy Archive” for pre-combined mosaics – they save time.

Troubleshooting

• Images won’t align: If TweakReg fails, try using more reference stars or a higher-order polynomial. Sometimes the distortion is not fully removed – use the “absolute” astrometric mode.
• Source extraction misses faint objects: Lower the detection threshold, but beware of false positives. Use a matched-filter approach or increase the signal-to-noise cut.
• Color image looks unnatural: Adjust the stretch – a simple linear stretch often makes the image too dark. Try an asinh or log stretch.
• Webb data is saturated in the core: Use the shorter exposure (e.g., the “short” dither) or mask the saturated pixels. For the very core, use Hubble’s higher resolution to complement.
• Flux calibration seems off: Verify that you are using the correct PHOTFNU or PHOTMJSR keywords from the header. If in doubt, check the instrument documentation.

Frequently Asked Questions

1. Why do we need both telescopes if Webb is more powerful? Hubble sees in optical and ultraviolet light, which traces hot gas and young massive stars. Webb sees in infrared, which penetrates dust to reveal the embedded protostars. Together they cover the full evolutionary sequence.
2. Can I use only Webb data? Yes, but you’ll miss the optical context – you won’t see the ionized gas or the scattered light from the outer disk. For a complete picture, both are ideal.
3. How do I know which filters to use? For star formation, use Hubble’s F656N (H-alpha) to trace ionized gas, and Webb’s F150W and F200W to see the dust-penetrating continuum. MIRI (F770W) is great for warm dust.
4. Is it hard to combine data from different telescopes? It requires some technical work (alignment, resampling), but tools like astropy and DrizzlePac make it manageable. The MAST archive even provides some pre-aligned products.
5. What is the biggest challenge? The biggest challenge is astrometric alignment – the two telescopes have different point spread functions and distortions. This is solved with careful calibration.
6. Can I study star formation in other galaxies? Yes, but the resolution is lower. You’ll see giant star-forming regions, not individual stars. Hubble can resolve individual stars in nearby galaxies like the Magellanic Clouds, and Webb can see through dust in more distant ones.

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