Surreal illustration of a pulsating red giant star in the Galactic bulge.

Red Giants in the Galactic Bulge: What High Mass-Loss Reveals About Stellar Evolution

Samir D’Costa in Science & Nature August 2025 • 5 min read.

"Uncover how extremely red stars are reshaping our understanding of stellar lifecycles and galactic composition."

In the heart of our galaxy, nestled within the Galactic bulge, lie ancient stars known as asymptotic giant branch (AGB) stars. These stars, nearing the end of their lives, undergo dramatic transformations, shedding substantial amounts of their mass into the surrounding cosmos. Astronomers have long been intrigued by these stellar behemoths, particularly those exhibiting extreme redness due to the dense clouds of dust enveloping them. This dust, composed of elements forged in the stars' cores, obscures their visible light, rendering them prominent infrared sources. The expelled material enriches the interstellar medium, providing the raw materials for the birth of new stars and planetary systems.

A recent study focuses on these extremely red AGB stars, specifically within the Galactic bulge. The research, led by F. M. Jiménez-Esteban and D. Engels, delves into the characteristics of these stars, challenging existing models of stellar evolution. Traditional models suggest that stars in a specific mass range should transition into carbon stars as they approach the AGB phase. However, observations of the Galactic bulge reveal a surprising abundance of oxygen-rich stars, defying these predictions. Understanding this discrepancy is critical for refining our comprehension of how stars evolve and contribute to the chemical makeup of galaxies.

The impetus behind this research was to understand the disconnect between the theoretical expectations of stellar evolution and the observational data from the Galactic bulge. By analyzing the spectral energy distributions (SEDs) of these stars and comparing them with stellar models, the researchers aimed to determine the mass range of the most reddened AGB stars and to investigate the reasons behind their unexpected chemical compositions. This exploration promises to shed light on the complex interplay between stellar mass, metallicity, and evolutionary pathways in one of the most densely populated regions of our galaxy.

How Do Reddened AGB Stars Defy Stellar Evolution Models?

Surreal illustration of a pulsating red giant star in the Galactic bulge.

The recent study employed virtual observatory tools to construct spectral energy distributions (SEDs) for a sample of 37 evolved stars in the Galactic bulge. These stars were selected for their extremely red IRAS colors, indicative of substantial mass loss and dust obscuration. Using DUSTY models, the researchers fitted the observational data to infer bolometric fluxes, which represent the total energy output of the stars. After accounting for interstellar extinction—the dimming and reddening of starlight by intervening dust—the team determined the luminosities and mass-loss rates of the stars.

The luminosity of a star is the total amount of energy it emits per unit of time. Mass-loss rate refers to the amount of material a star ejects into space over a given period. These parameters are critical for understanding a star's evolutionary stage and its impact on the surrounding environment. In this study, the observed SEDs were mostly consistent with classification as reddened AGB stars. The study revealed the following:

Luminosities: Ranged from approximately 3,000 to 30,000 times the luminosity of our Sun (L☉).
Mass-Loss Rates: Varied from 10-5 to 3 × 10-4 solar masses per year (M☉ yr-1), indicating rapid expulsion of stellar material.
Mass Range: Estimated to be between 1.1 and 6.0 solar masses (M☉), assuming solar metallicity.

These findings presented a puzzle. According to stellar evolution models, stars in the mass range of 2.0 to 4.0 M☉ (for solar metallicity) should have already transitioned into carbon stars during this stage. Yet, the observations revealed a prevalence of oxygen-rich stars within this mass range, contradicting the expected chemical compositions. This discrepancy suggests that factors beyond stellar mass and age influence the evolutionary pathways of AGB stars in the Galactic bulge.

What's Next?

The future of AGB star research involves more in-depth study of chemical composition, spatial distribution within the Galactic bulge, and comparison with stellar populations in other galaxies. New details will likely provide the answers needed to help better understand how stars evolve and change the galaxies around them.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1051/0004-6361/201424609, Alternate LINK

Title: Study Of Extremely Reddened Agb Stars In The Galactic Bulge

Subject: Space and Planetary Science

Journal: Astronomy & Astrophysics

Publisher: EDP Sciences

Authors: F. M. Jiménez-Esteban, D. Engels

Published: 2015-07-01

Everything You Need To Know

What are extremely red Asymptotic Giant Branch (AGB) stars, and why are they important for understanding the Galactic bulge?

Extremely red Asymptotic Giant Branch (AGB) stars are stars nearing the end of their lives that exhibit significant redness due to the dense clouds of dust enveloping them. This dust, composed of elements forged in the stars' cores, obscures their visible light, making them prominent infrared sources. They're important because they challenge traditional models of stellar evolution, particularly concerning the expected transition to carbon stars. Studying these stars in the Galactic bulge helps refine our understanding of stellar evolution and the chemical makeup of galaxies.

How does the mass loss from Asymptotic Giant Branch (AGB) stars contribute to the interstellar medium and the formation of new stars?

Asymptotic Giant Branch (AGB) stars expel substantial amounts of mass into the surrounding cosmos. This ejected material enriches the interstellar medium with elements forged in the stars' cores. The enriched interstellar medium then serves as the raw material for the birth of new stars and planetary systems. This process is crucial for the ongoing cycle of star formation and galactic evolution.

What discrepancies were found in the Galactic bulge regarding the chemical compositions of Asymptotic Giant Branch (AGB) stars compared to theoretical models?

Traditional stellar evolution models predict that stars in a specific mass range (2.0 to 4.0 solar masses for solar metallicity) should transition into carbon stars during the Asymptotic Giant Branch (AGB) phase. However, observations of the Galactic bulge revealed a surprising abundance of oxygen-rich stars within this mass range, contradicting these predictions. This discrepancy suggests that factors beyond stellar mass and age, such as metallicity or other unknown parameters, influence the evolutionary pathways of Asymptotic Giant Branch (AGB) stars in the Galactic bulge.

How did researchers use spectral energy distributions (SEDs) and DUSTY models to study extremely red Asymptotic Giant Branch (AGB) stars in the Galactic bulge, and what parameters did they determine?

Researchers used virtual observatory tools to construct spectral energy distributions (SEDs) for a sample of evolved stars in the Galactic bulge selected for their extremely red IRAS colors. They fitted the observational data with DUSTY models to infer bolometric fluxes, which represent the total energy output of the stars. After accounting for interstellar extinction, they determined the luminosities (ranging from 3,000 to 30,000 times the luminosity of our Sun) and mass-loss rates (ranging from 10-5 to 3 × 10-4 solar masses per year) of the stars. This analysis helped estimate the mass range of these stars, found to be between 1.1 and 6.0 solar masses, assuming solar metallicity.

What are the implications of the observed mass-loss rates of extremely red Asymptotic Giant Branch (AGB) stars in the Galactic bulge, and what further research is needed?

The high mass-loss rates observed in extremely red Asymptotic Giant Branch (AGB) stars in the Galactic bulge (10-5 to 3 × 10-4 solar masses per year) indicate a rapid expulsion of stellar material. This significantly impacts the chemical enrichment of the interstellar medium and the evolution of the stars themselves. Further research is needed to understand the chemical compositions, spatial distribution, and comparison with stellar populations in other galaxies to better understand the factors influencing the evolutionary pathways of these stars.