Earth-like planets, dark energy and variability of fundamental physical constants can be discovered by observing wavelength shifts in the optical spectra of astronomical objects(1-5). These wavelength shifts are so tiny that exquisitely accurate and precise wavelength calibration of astronomical spectrometers is required. Laser frequency combs, broadband spectra of laser lines with absolutely known optical frequencies, are uniquely suited for this purpose(6-13), provided their lines are resolved by the spectrometer. Generating such astronomical laser frequency combs ('astrocombs') remains challenging. Here, a microphotonic astrocomb is demonstrated via temporal dissipative Kerr solitons(14-16) in photonic-chip-based silicon nitride microresonators(17), directly providing a spurious-free spectrum of resolvable calibration lines. Sub-harmonically driven by temporally structured light(18), the astrocomb is stabilized to a frequency standard, resulting in absolute calibration with a precision of 25 cm s(-1) (radial velocity equivalent), relevant for Earth-like planet detection and cosmological research. The microphotonic technology can be extended in spectral span(17,19-24), further boosting the calibration precision.